As you are UK based then PC Pro's laptop reviews
may be of interest. As you'll be running CS5 you'll want to avoid anything with a low powered processor (IMHO anything with an "Atom" processor is an absolute no-no) and make sure you get plenty of RAM.
I've had a few Thinkpads in my time, though none were used for photo-editing, and I've been pleased with them. With that in mind I note that PC Pro gives the X100e a generally positive review
but I'm not sure that having a "single-core 1.6GHz Neo processor" is going to be quite up to the job. However, if you check out the Lenovo Offers website
then, as I write, they are offering the X100e with a dual core processor for £414 including VAT - hopefully this link
will get you to the right page. This still isn't the fastest machine on the planet but for the price it seems to have a pretty reasonable screen compared to some of the opposition at that price point and might do providing you don't expect miracles. If you are tempted by this machine then an extra £25 buys you 3GB of memory (default is 2GB) and an extra £12.38 ups the HDD from a lowly 160GB to a more reasonable 320GB.
If you shop around you might find the same machine for a similar price but you'd need to be very careful about what upgrades, if any, have been specified. For example, the single core CPU version is a lot cheaper but it will run CS5 a lot slower as CS5 is one of those programs which will actually use all the cores (and memory) you can throw at it.
I'm sure you'll have other suggestions but I'm very concious that at your budget there's a significant danger of either compromising the screen quality to get the processor speed or vice versa. I've not used the X100e (in either version) so if you do think it's a posiblle solution for you I'd still urge you to do some more research on the Net and try and find some reviews, particularly group reviews of laptops at that price point. Another possibility might be the ThinkPad SL510 with discrete graphics
- it has a larger screen but I get a sense from a quick scan of the reviews that the image quality isn't quite so good as the X100e. But it does have a faster processor, you can opt for a 64 bit version of Windows 7 at no extra cost and that would allow you to fully use an expansion to 4GB of memory (only £33) and an upgrade to a 320GB hard disk drive only adds another £6 to the bill. Certainly worth another search for some reviews, I think.