This may be a silly question but..
In the context of US firms, when talking about the work they do, would you focus on the work that the London firm is known for?
For example, if a firm is known for its corporate takeovers in the US but is only band 3/band 4 in London, is it not worth mentioning as I will be joining the London office?
Is it also redundant to talk about deals that the London office did not work on?
It would depend on the context of what you are saying. If the London office collaborated and supported the US side on a deal and you're interested in that sort of work, then it could make sense to bring up. If the London office had nothing to do with it, then it would be redundant.
More generally, and especially if there is a lower word count, you probably want to focus on what the London office is good at and known for. Although, growing areas of practice currently band 3/4 can also be interesting to mention from a strategy perspective.