If you go to any nationaly affiliated bank, they can issue you a cashiers check in pounds - english currency. You can pay in cash. They will convert it.
Ask to see a bank officer. Not everybody who works in a bank will know how to do this... but somebody will know!
If you get a check in pounds, the same person will know how to convert it and cash your check. There may be a delay...depending on how fast they can verify it.