I'd be much more worried with a 1 hour connection in the opposite direction. The UA flight to FRA will park at the terminal, rather than a remote spot with a bus ride. A BHX-FRA flight will stop at a remote hardstand, and require a bus ride. There is no significant security delay arriving from the US, while the trip back to the US requires an additional (and inefficient) security check. Depending on where UA 900 arrives, it might be a long hike to the FRA-BHX gate, but you can check the gates using lufthansa.com. Make sure you get a boarding pass for the FRA-BHX flight when you check in at SFO.