Virgin used to be the work phone SIM of choice, with no line rental and credit that lasted beyond 30 days. Others have caught up with that (most of them anyway, I think you have to use up credit on Three quickly) but I am surprised they want 25p/min off you.
The big question is how many calls / texts / internet do you make / send / use? If it's low, then I still have Virgin and my bi-monthly bill is about a quid.