Well Eddie... I only know of two phone companies in the UK, and those are NTL and BT. Personally, if I were you, I'd pick the cheaper.
BT is extremely big in the UK, so obviously has lots of clients and therefore you'd think has a good service offering.
Maybe try phone a company like Jungle.com or someone, get through to their Customer Relations dept. and then ask them who they use for their 0800 number - see what they say.
They may turn you away, but there's not much you can really lose, is there?
I'd go with BT in all honesty, they are big, therefore should have the cheaper prices, and also are the biggest in the UK for telecommunications.