network advice needed

Hi,

I am thinking of buing a Sony Ericsson mobile (probably w200i).

I will be using the phone only occasionally. Can anyone advise on
networks. I would like to make the cheapest uk calls. It is also
important that the phone credits do not expire (I will be using 'pay
as you go')

I am new to mobiles. Can I top up without a credi/debit card?

Thanks