Try eBay, there are loads on there, chances are you can find a buy it now ad within London! Unfortunately everyone buys their beer brewing stuff online in uk pretty much.. If you've got time and you're able to take a mail delivery you can try Colin farrar brewery supplies for new kit.
One thing if you are buying a cask beer engine, you have to make the controversial decision whether to go sparkler or no sparkler. I personally prefer without as I'm a southerner! Good luck with it!
Would be v interested if someone else has a better suggestion than eBay.