Hey michey_b,
I don't *think* you'll have any trouble using your UK credit card to buy air tickets online or by phone. Have you tried? (Greyhound is a whole 'nother animal, and shouldn't be confused with an actual company.

)
As far as getting the lowest fare....well, welcome to our version of hell. Easier to offer help if we know where you're traveling from and to. In
general, I go to travelocity and punch in a variety of dates for the airports involved. This gives me a decent overview (minus a couple carriers) of who flies there, routing, and price variations. Often, the cheapest tickets involve crap routing, long layovers, multiple stops, etc. Once I've determined the best dates (if flexible) and desired routing or carrier, then I'll go to that airline's website and price it there. I fly Alaska a lot, and it's usually about $10 or so less buying directly from the airline website.
Worth checking the airlines travelocity doesn't include (as static mentioned, Southwest, Jetblue and maybe Spirit?) if you know they also serve those airports, or another airport nearby.
Other than that, I've largely given up trying to beat the system. It's just too confusing and I end up wanting to commit harikari.
Good luck!
P.S. Of course, Boots also sells tickets....(sorry guys!)