I have noticed that O2 & T-Mobile do not charge for buying the phone. However i have noticed that 3 & Vodafone do (3's is £49 and Vodafone's is £59). Note that i haven't looked at Orange cause i don't like them.

This one off charge, i take it you pay that at the end of the purchase straight off your credit or debit card (in my case, debit card).

Also when does the first monthly charge get taken?

O2 is the cheapest for the 3GS £21.50 if I remember, First months payment is taken 10 days after you sign up. However O2 are the worst for 3G coverage in the UK but probably the best for 2G