I sometimes think it's best to pay a little more if you can - there's usually a catch with so-called 'low-cost' ISPs (crippled speed, limited access times, long contracts, high contention).
I use
Zen Internet - small company based 'oop North - always up, always quick, with good quality customer service and friendly support. Price for 512k is £23+VAT per month, modem self-supplied and installed.
They provide one-month contracts and (when I signed up at least) didn't charge a connection fee.
For anyone thinking about switching to broadband, I'd recommend looking at
this site first.
Jx