This is more of an option than an answer to your question, but I recommend safari.oreilly.com for tech books. I'm assuming it is available for our UK brethren.
The reason this is more of an option is because you don't buy the books, you check them out much like a library. For about $10 US (which is the low-end plan--they have more expensive options), you can have 5 books checked out at one time. You have to keep the book checked out for one month, but after that you can keep it or check it back in and get another one. Thing is, though, is the books are only available online. You don't physically get the book, but it is presented electronically just as it would be if it were printed.
I love it because my interest and needs change regularly, and once I have what I need from a book I don't have it laying around collecting dust. Also, it's a rather earth-friendly way to learn since there is no paper wasted.