There are no F/A-18s based in the UK, but they could be visiting from other European airforces (Spain, Switzerland, Finland as far as I'm aware). USAF currently operates F-15Es out of Lakenheath, which makes those seem more likely.
Just read that the US intends to take its Strike Eagles home...
Exactly what will replace the forward-deployed F-15Es, which are some of the most in-demand aircraft in the USAF, at RAF Lakenheath isn't clear. The Air Force plans look likely to see RAF Lakenheath become an all-F-35 base.
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