Xp100's don't exist over this side of the pond. I'm happy fabricating a trigger mech and making a stock but is there any recommendations for an action to use?

I'd be going for a .308 and making it a single shot. The Rem 700, as always, is looking like a good donor.
Anything like this been done before?

I haven't personally but so know of a fella who did and used one of these Choat sniper stocks and cut it off behind the grip.

http://www.riflestock.com/store/do/product/SPS/22-01-09

Can you find a Rem. 600 or 660 action there?

Rem Model 7 would work as well.

Unfortunately 600 and 660's aren't available here and model 7's are in on very short supply. We can get the new ones but they aren't cheap.

I'm tempted by a Savage 110 for ease of barrel swaps in the future.

Can you get the single-shot target action?

Not without buying the entire rifle!
http://www.guns.gb.com/contents/en-uk/d470_Savage_Arms_model_12_precision_target_series.html

We had a few actions come in but at the price they were you were close to a proper (700 footprint)custom action.

Looked at a Winchester M70 Featherlight today which looks like it would work nicely with its minimal tang. as does the Howa 1500 and atleast i can get that as just new a barrelled action.

Is it legal there to take an existing rifle action and turn it into a pistol?

Yeah, I know you guys cant do that unless the action is a pistol from the factory. Our licenses have 'slots' so for example mine will say '.308 Long Barrelled Pistol'.
I can't buy a rifle with that on my license but I can buy a barreled action without the stock and the serial number of the action goes on my license as a LBP.
Its a bizarre system but since when has firearms licensing ever made sense!
We can also have moderators (suppressors) here without that stupid tax stamp you guys have to pay. However, moderators are counted as firearms so you need a slot on your license for one!
Most hunting rifles here are moderated.

Are you right or left-handed when shooting?

Its a long story but basically I am right handed when I shoot. To be honest I've learnt today my local gunshop can get me a Bergara (CVA) Apex for a great price so for the ease of it I may just get one of them.

It would be a lot cheaper that way.
I would go with a LH Remmy SA ideally,
Have a XP McMillan stock modified to handle a longer action, modify XP trigger linkage, single-shot follower, etc.....
Or purchase a custom LH single-shot short action.
Still a lot of work and it would be pricey

I think I'll go Apex for my first one. Make a pistol grip and matching forestock, cut and thread the muzzle for a moderator/muzzle brake and then go shoot it. Getting one in .308 is tricky so it's .270 or 30-06, I'll probably go 30-06.
Should do me nicely for Africa aswell.

The bolt action, mid grip can be in something a little more potent for larger game.

In a bolt rig a Big 7mm, 30, or 338 based off of a Dakota case or something similar will give you all kinds of performance.

Jim, I have enclosed a book picture of a "Shield Long Range Free Pistol" made by one of your country men. Say`s it was made using a Mauser action in 7.62 NATO. Using a 168gr Sierra Match King over 35 grains of Reloader 7 it would stay within 3 inches at 200 meters. The book is titled "Modern Sporting Guns" and published in 1988. The section showing this handgun is called "Target Pistols and Revolvers". It is a most interesting book with a strong English flavor. The bottom picture is another Shield Long Range Free Pistol single shot in 357 Mag. Both pistols were designed and built by Andy Wooldridge of Shield Firearms. The founder of British Long Range Pistol Shooting was a gentleman named Gillie Howe, a much decorated color sergent of the Royal Mariens who, the story goes, used to while away his time on often tedious NATO postings above the Arctic Circle by plinking across the tundra with his issue Browning Hi-Power. The futher he shot the more he became fascinated with the question of long range handgun marksmanship hence his founding of Long Range Pistol. With all the restrictive gun laws in England I don`t know if Shield Firearms is still in business, however, if they are still around it may be a good place to get your questions answered. Best of luck with your handgun project......

I thought that all handguns were illegal in England.

Ernie, I'm a big bite kind of guy so I was thinking .375 and up

should give me something to focus on when I touch one off!

James, what a fascinating post, thank you very much. Great read. Long range pistol shooting still exists here but it has now changed in its format to become an F-class of sorts. See here; https://m.flickr.com/#/photos/762_photo/sets/72157618922295155/

Wheeler, that's a common misconception the same as suppressors being banned in America. We are heavily restricted but basically we can have handguns which are 24" overall length and with a 12" barrel. So what you end up with is 12" revolvers/single shots/semi autos (the last one in .22lr or wmr only) with a thin metal extension out the back grip. Google Long Barrel Revolver UK to see what I mean.
We can also have normal revolvers and derringers for humane despatch of deer but we can only have them for that purpose bizarrely.

I believe this is a UK SRH in 357 mag.

It is indeed. River built a bunch specifically for us. They sold like hot cakes but Ruger won't make us any more.
I believe it's a 20" barrel, many had the extension added to the grip and the barrels chopped to 12".
If I found one in 44 Mag I'd be all over it!

This is how they look when chopped http://www.shootforum.com/forum/viewtopic.php?f=157&t=13265

Sorry, having trouble with posting just the phots

A 375 would be dependent upon what case you wanted to start with.
The 284 Win case, 30-06 case ( I would start with the 35 Whelen case myself), 35 WSM, and the 330 Dakota case.
I have a 375 bore SP, but it hold around 150 grains of powder and it is not a XP action, but a larger single-shot action and it is as heavy as a pig.
I have the 338 Lapua Improved in the XP action and several 7mm Dakota's.
If you want to skip wildcatting, then you have a number of cases to consider:
375 Dakota, 375 Ruger, 376 Steyr, and 375 RUM in no particular order.
Guessing the 375 Ruger may be about the cheapest in terms of brass.
The 376 Steyr may be easier for you in terms of brass???

Sounds like some proper animals you have there! The 338 Lap Mag Imp sounds like a great long range rig. Little jealous if I'm honest.

Wildcatting is a definite interest of mine. I've designed a sort of super .44 myself. Quick load reckons I should be able to load .44 Mag and 444 Marlin bullets at their respective velocities with reduced loads, perfect for deer, and propel heavy paper-patched/GC hard cast at very respectable speeds. I feel that it may be a handful with the top loads

Maybe a 470 Capstick then in a XP is what you are looking for????
I know a guy who has one