I do have some small experience with Hood building in Celtia and may be able to answer your question, though I was not involved in the decision-making process of creating Caer Lud.
I do believe the reason Celtia does not have a Londinium is precisely because there is one in Rome, and also because it was a Roman city, not a Celtic settlement. The original Hood creators were probably trying to avoid redundancies and historic inaccuracies.
Far as I know, there is no written record of Londinium ever being a pre-Christian Celtic city, even after it was abandoned by Rome. And it was marshland before the Romans, and, therefore, did not likely have any Celtic settlements. Well, maybe a few crannogs, but none called Londinium. LOL
You will also find a Dublin in Germania, but not in Celtia. That is because it was a Viking settlement founded after the AW timeline in Celtia, which is the pre-Christian, per-historic era, i.e., before written history, which in the UK and Ireland, was before the Celtic Christian monks started scribbling notes in Gaelic in the margins of their Latin manuscripts.
This could easily change if members are interested in building Londinium or Dublin in Celtia. Hood building is a fluid, ongoing, growing project. And who knows? Some day the old Hoods may even be rewritten and presented in a more interesting and involving format.
If a member has ideas for the creation and building of a Hood and is willing to take on the responsibility of the research and writing of said Hood, I would suggest contacting the Scribes of that world and running your proposal by them.
At least, these are my thoughts, for whatever they're worth. *S*