Loksound are very good, I have a 4884 Big Boy that came with one fitted, best steam rendition Ive heard yet.
I'd also say the QSI decoders, especially the new revolution, which is, I think, more advanced than the loksound, but restricted to US outline stuff so far, as its a very new product.
Like loksound they also sell a reprogrammer (about £60 from digitrains) so you can change the sounds in them , its very much like the latest Zimo ones, in that you can cut and paste individual sounds into a project to change just the horn (or whistle) etc.
I have one in an athearn F series diesel, a US outline model, and its fantastic, but as said, someone will have to start making the sounds for the UK outline stuff for it to be a viable option, price wise its not much cheaper than loksound really.