Suggestions For A Londinium Mint Purchase?

Well they're a little more scarce than most of the other mints (but not difficult to find), so if there will be anyone selling ancients there it isn't a guarantee they'll have a London mint coin to sell. With ancient coins its much easier to find what you are looking for online, since the vast majority of coin shop/show dealers don't carry them.

While the Constantine I and family issues will be easiest to find of the London options, while shopping be on the lookout for a possible London mint coin of Carausius. He was the usurper that is most associated with Britain and not all of his coins are London mint but it would seem that someone looking for a coin that is special from London might want one. I would suggest buying a nice Constantine if you see it and not hold out for the Carausius which might be harder to find (or afford) but is is something to be on the watch for in the search.

If your interest is UK rather than just London, Carausius also issued coins from Camulodunum (Colchester).