Fidelity
I use Fidelity. The charges are low and there is a wide range of funds to invest in (spread the risk in a balanced portfolio). Remember you can put £7k p.a. in to an ISA which is tax free if you are UK resident.
You can also switch between funds, so if there is an area or sector you get a tip for, switch.
Markets have rallied a bit since the sell off in May, so don't pile in now. I try and buy after a market has fallen (they usually do at some point), but a drip feed in every month or so is a way of evening out the peaks and troughs.