... but Elaine, please see erinb's original posting - calling for "... dress trouser outfit" - just what exactly is that - if I saw one on the street, would I know? Is it fashionable?

Face facts: it is as much as HOW you wear it and how you carry yourself as it is the clothing" itself. Self-confidence will go a long way and I kinda doubt anyone is going to throw you out of a place. What is "smart" and "fashionable" is very subjective...and you certainly don't need to be wearing "heels" for tea at Brown's (that's pronounced "Brines" by the way). And I agree that you don't need to dress way up for London theatre.
Somehow I think you'll look stunning no matter what you decide and since you don't know what anyone HERE looks like when they are "dressed up" I guess you'll have to do a certain amount of looking into the mirror and confirming what you already know: "I look GREAT."
Enjoy London.

By God < Intrepid's got it!

Yes ...it does depend on attitude and self confidence! One can be wearing heels and the best outfit and still look like a schleep, if they don't have confidence in their appearance!

Be comfortable with yourself first!

Smart casual means American investment banking dress down Friday dress. Wlak down Wall St or into the City on a Friday (or during the dotcom era on any day) and you will see thoudsands of workers dressed the same (ie, in khakis, a blue collared shirt, and a piar of brown leather shoes). So long as you look smarter than all these Wall St and City idiots who all dress the same you have nothing to worry about.