Peak is excellent. I've used them many times in the past, and have always been happy with their service. But nowadays, I send all my film to Genie Imaging in Wandsworth, South London.
They have an ideal (to me) combination of quality, price (especially!) and turnaround time. Plus they are local to me, and that helps as I'll explain below.
They charge £1.99 per roll + VAT for processing unmounted slide film. I'm pretty sure they'll do mounting, for an extra charge. They usually have your film ready within a day or two, and they use dip & dunk processing.
Where they can be a bit more expensive is postage--they send your film back signed-for which costs nearly a fiver. However, as they're right by Wandsworth Common Station, I just drop the film off and pick it up when it's ready.
If you're on a budget, give them a try. They do an excellent job and are committed to helping keep film alive.