Off the top of my head, it could be that the base of the airway is lowered to assist descent into the Mancheter TMA , inside which the Transition Altitude is 5,000ft. I believe REXAM is also used for EGCC STARS.
The base of some of the lower airways are also defined as an altitude
as well as a Flight Level for use when the QNH/RPS is lower than 1013.2. The altitude is given to show the lowest possible level of the base of the airway (i.e FL45 would be lower than 4500ft and possibly not terrain safe.)
Hope that makes sense..........
