It can be done direct from BRS at a push, and was on one occasion last year due to an 8 hour tech delay, they operated direct to avoid further delay, one major reason for them going via MAN is for cargo reasons, they whack a load of it on up there as it makes the airline mega bucks more than swallowing up any extra cost to go via MAN.
Also from a CWL point of view, dont forget the BRS longhaul programme was set up by First Choice not Thomson, before the merger, and obviously FCA were more pro BRS than CWL.