Has anyone ever purchased London theater tickets from the US? If so, is there a particular service that is better (or worse) than another?

I know that tickets for most shows are readily available this time of year, but we are limited in our flexibility so need to purchase in advance.

FYI - BA is offering some almost too good to be true deals. (hope that's not a commercial plug!)

Any suggestions would be appreciated!

Thanks, Vanessa Cutchogue, NY