First, THANK YOU!
I've read and re-read everyone's comments a number of times, slowly realizing that most of the questions I left unasked were just anxiety about being clean and relaxed enough to enjoy ...
As for the advice - @oldNbusted handled the majority of questions with one link (and thank you for that). Reading @MassageDrew and @NuSingle reminded me that the rare bottom experiences I had, remember and fantasize about, started with (copious alcohol and poppers, and...) me on my back or spooning.
@NuSingle, thanks for the comment about poppers. I came out in New York and Houston in the late 70's, poppers were integral to my coming out experience. A number of brands available in London are still quite good.
I also realized that the phrase "size inappropriate" left too much to the imagination... The escort I'm hoping to start with next week advertises as 8 inches, and it was looking at his advert that prompted my question. There is a guy in Paris I sometimes fantasize about who is 9 inches, but I think of him as a very advanced classes best left for the future.
finally, THANK YOU - I'm very much looking forward to next week.