This question seems harder than "what is the meaning of life?" on the internet. If I was to walk into the Apple store in the UK and ask for an iPhone 3G (including saying that I'll activate the contract on iTunes at home - yes you can do that), what would the price be of my purchase?

For this situation lets use the 8GB but if someone could tell me the 16GB as well it would be appreciated. ALL ANSWERS WILL BE APPRECIATED, you don't realise how challenging it is to get such a simple answer. UK Apple stores/prices only thanks!

The answer is easy, the price and tariff is the same whichever store you go to in the UK, The price is fixed across Apple, O2 and Carphone Warehouse. You're not using Apple as a service provider much the same as Carphone Warehouse so the tariffs come from O2. PAYG is not available yet so you'd have to go for Pay Monthly, tariffs can be found using the link below. Hope this helps clear things up a little
http://www.o2.co.uk/iphone/paymonthly