What is the best desktop computer on the market today in the U.K ?

The best thing for you to do is decide what you need this computer to do for you. Do you need lots of storage? Do you plan on using this computer for working on photos? Is it to be used for college work? Do you need a web cam? Do you need a card reader and for which cards? Do you plan on doing any gaming? Once you answer these questions about your needs and you know your budget, you can enter the statistics for a computer in your price range, and start to gather information on what's available so you can get the most value for your money. You can look at the stores you mentioned web sites to see what they offer. You said you're buying this computer tomorrow, so you're not allowing yourself much time to decide! If you start shopping uninformed, the sales people will try and sell you their best "profit maker", and it may end up not being what you want. Get educated on what you need in your computer and what items are in each computer you look at. RAM is very important. The processor(s) and their speed is important. The operating system and software programs included may be important. Do some research and go shopping with some goals in mind! Good luck with your new computer!