The very last airliner built in UK was the second RJX airframe, which never went into pax service after the RJX project was cancelled. That aeroplane is now at Manchester Airport viewing park.
That was not the last RJ/RJX to be built. The RJ was still in production while the RJX was in development. The last RJX's first flight was on 09-01-2002 but the last RJ85's first flight (the one referred to above) was on 26-04-2002.