Very happy with TalkTalk myself (no I'm not a plant). They're by far the cheapest (£15.70 + £3.75 if you
pre-pay the line rental and you can probably get a free year.. saving you the £3.75 a month).
There's reasonable support via their Community forum. Just don't try to call/talk to them
Talk, talk all you like but they're not listening.
I have 6 VoIP numbers (via Sipgate) and they all work fine. You can probably keep your old landline
number as well.
PS Recommend using your own router (Draytek), the ones I've seen from ISPs aren't that stable.
I do know someone who recently switch from TT to BT because of the supplied router, too short
sighted to see the benefit of having/investing in their own good kit.