mod it yourself! i recently bought an xbox and a xenium ICE mod chip (only 3 weeks ago!). i did a lot of research on xbox-scene.com, and i decided to get it modded form someone in my area. when someone on the forums told me to do it myself, i gave it a second thought, and i decided to mod it myself. sure it took me a long time since i'm relatively new to soldering, but i successfully modded my xbox on my first try. it isn't as difficult as you think, and to me the experience gained, and the satisfaction of doing it is well worth the time. if you have the time i highly reccomend doing it yourself. not to mention you save a bundle of money, money that could be used to buy a new hard drive for the xbox (which is what i did).
if you stilll want to buy one premodded you can try bmmods. their site is reliable and has good reviews. <edit> although i'm not sure it's gonna help since you're in the UK. <edit>
well, a mod chip basically allows you to replace the default retail xbox bios which can only play signed code with another bios that will allow you to run unsigned code (basically anything you want). the functionality of most mod chips is exactly the same, the interface and installations are a bit different. i have a xenium ice and i love it, although you may want a xecuter 3. i would do more research on xbox-scene to figure out which one really suits your needs. it has comparisons, and the people on the forums are helpful. remember, when you get down to the nitty gritty, any mod chip should be able to run any bios. you can run the cromwell/evox/xecuter 2 bioses on any mod chip (this is fact). i'm not sure you can run the xecuter 3 bios on any mod chip though.
I got one but i am US. Best place is ebay, i get the ones with bad dvd drives, and since i have like 4 around almost all the time, i just swtich it out and have a new one for like $60. its a great deal.