If you're not scared of a little noise then you can pick up a non airbrush compressor for less money. They typically have larger tanks and more powerful motors as well as being cheaper. The drawback being of course that they are much louder as they're designed more so for outdoor use.
I can't advise you where to get one in the
UK but here in the states they're in every home improvement store and the ubiquitous Walmart starting at about $50.
Edit:
Quick Google-Fu found this:
http://www.amazon.co.uk/Wolf-FOX-Compressor-Litre-7CFM/dp/B00BIJ2VK4/ref=sr_1_48?ie=UTF8&qid=1365047949&sr=8-48&keywords=air+compressor That's the kind of compressor I'm talking about. I use one similar and haven't had any problems though I just got into airbrushing myself. It's also handy as it can run a variety of air tools for projects around the house.