All the lens I have listed will give you a good improvment over your current lens.
Primes offer the best performance but are less flexable.
Try your current 18-55 set to 50mm and dont move the zoom, see if you can get on with a fixed length. If you are happy with a fixed 50mm go for a prime.
The Tamron 17-50 f2.8 is a very good lens, fast glass and great sharp images best of all is very cheap too.
For your £500
you could get a mint 2nd hand tamron 17-50 f2.8 and a canon 50mm f1.4 or maybe if you are lucky a sigma 50 f1.4
I think if I were you I would sell my current lens and buy 2nd hand tamron 17-50 f2.8 and canon 70-200 f4. Then do a bit of saving for a sigma 50mm f1.4. this is will improve all you images, but its all down to choice.