>>6362386 April RAF to LA query.
Look up what a RAF aviation callsign looks like.
Go to flightaware.com
Type or click the desired LA area airport.
Request historical landings data.
Start scanning callsigns for a UK callsign.
Request next page.
Rinse, repeat.
Next day. Rinse repeat.
Next airport. Rinse repeat.
It's not quite a needle in a haystack problem but almost, with the tools available to us.
This tool will have civil air traffic but not military.
If it landed at a military air field then you pretty much need to know the tail number to look up its activity using ADSBexchange.com historical flight data.
It's not a task I would volunteer for.