Vicki, the rule for a/an, in UK as in US English, is pronunciation. If you say "you-neekly", then it's preceded by "a"; if anyone says "oo-neekly" then they should use "an".
By the same token, only people who say "'istorical" (and some educated speakers do) ought to talk about "an historical fact".
Going back to prepositions at the ends of sentences - that's one of those "rules" that was made up by the crazy Latin-loving Victorians with nothing better to do. People have always, before and since, separated prepositions from their objects. But, as a rule of thumb, either say "by whom" or "who... by" - using the formal "whom" with the less formal trailing preposition will get you funny looks anywhere in the English-speaking world.
The idea of a solicitor coming to the door (to force you to write your will, or what?) still tickles me pink.
Mere
A diabolically, fiendishly clever mind. Possibly someone evil enough to take over the world. CC Aiken, Can You Guess the Writer? challenge
People have always, before and since, separated prepositions from their objects. But, as a rule of thumb, either say "by whom" or "who... by" - using the formal "whom" with the less formal trailing preposition will get you funny looks anywhere in the English-speaking world. 
