Yes, you can use any wifi you want.
With a contract iPhone, you get unlimited 3G usage, as well as unlimited wifi usage from The Cloud, for the lifetime of the contract.
Because this is a pay as you go phone, you can do what you like with it, without paying a contract. They give you free usage of the 3G and wifi from The Cloud for the first year, then start charging you �10/month after this first year.
During any of this time, you can connect to any other wifi network you like. The Cloud may very well not be available where you live, but only in city centres or near certain shops.