Lumen, the abbreviation, for lawyer, isn't British, but pretty well known. I got the internal part, going inside the abbreviation, from Bradford's Crossword Dictionary. The clue definition works perfectly - though getting 10 early helped.
Yes, not as scary as it looked on reading the preamble. All done apart from 18, which I can't parse (and there are two possible answers). As far as I can see it's a clue with a word missing, and I know which one ... but beyond that I'm stuck.
I've got a full grid with the extra (third) letter question spelt out and the lines drawn illustrating the omitted word but I don't get what P and Q has to do with it, but nobody else has questioned this so clearly I'm missing something obvious?