What's your budget? Bathrooms at source is in a reasonable price range: not too cheap like Victoria Plumbing, but not as expensive as CP Hart or West One Bathroom. Unlike the others, it also has its fitters, which I think is good if you don't have a builder you trust (so you don't have builder and shop blaming each other).
If instead you are looking for an architect / designer that designs everything from scratch, shows you 3D renderings etc, then I'm afraid I wouldn't know anyone to recommend.
My two cents is that spending too much on a kitchen or a bathroom doesn't increase the value of a house, so you should do it if you like it, not in the expectation you'll get the money back if you ever sell the property. If you spend £30k on a bathroom when £10k could have got you a very nice one, I doubt you'll ever recover the extra £20k.