Well lets put it this way.
BA has made provision for 35 Man Power Equivalent spaces at the new LGW Fleet who will be from WWLGW. These people will accept a "buy down" of their current contract and go onto the nearest pay increment in the new fleet within their rank (CSD would become CM), and keeping their seniority. They would then accept being on the new LGW fleet T&C's.
As you can see BA has not anticipated there to be a big take up of this offer. The union echoed this sentiment.
The majority are expected to request transfer to WWLHR. All requests will be accomadated. To make this easier opportunities for part-time and unpaid leave will be made available.