'Which is the best AMD or Intel'...you will not notice any difference. Go for the cheapest...AMD.
'What should i be looking for Size wise'...2Ghz+ processor will be more than adequate. You will need 512Mb RAM though if you going to do photo-retouching although 256Mb will do.
£595 ex VAT (I assume that you will reclaim the VAT.....click here
Check out PCWorlds Compaq and Emachines....click here especially the EI SYSTEMS 5030
You should not need to spend over £700 incl. VAT to get a computer that is excellent for your needs. Don't buy an AV or firewall, ask here for free, well respected ones ;-))