No need for an arbitrary definition -- the British Invasion ended with Hendrix's emergence. It goes from 64-67.
http://en.wikipedia.org/wiki/British_Invasion
I lived through it, so I can validate that. By '67 the novelty was worn off that the influence was flowing the other direction again, as evidenced by Jimi's emergence.
I don't associate Pink Floyd with the British Invasion, even though they were around in that time -- mostly because they didn't really have much impact over here until after Hendrix. But I have no objection to including Richard Wright in the debate if you guys want.
But the best keyboardist IMO of the British invasion --
Alan Price of the Animals.
I'll take Alan's drama and melodic construction over Argent's noodling any time.
Runner up -- Stevie Winwood.
And if Wright can be included, then I can add Matthew Fisher of Procol Harum.
So:
- Alan Price
- Stevie Winwood
- Matthew Fisher
Honorable Mention -- Mike Pinder of Moody Blues