can you use your own wi-fi on iphone 3g?

i want to get the new iphone on pay as you go… (i am in UK!)
i want to know, can i use my own personal wi-fi to access the internet because i've seen you have to pay like £10 a month for it and you can only download like 5mb a month or whatever.
and i don't wanna pay! or is what i just said for O2 hot-spots that you have to pay?