Cabin crew sickness has long been a problem at BA LHR. It is certainly not helped on the Worldwide fleet by the fact that there are wild variations in the allowances paid on routes (hence the terms "charity" and "money" trips) and that there is no bidding system to allow crew to bid for trips based on personal preferences etc.
Replacing the archaic and utterly uncomprehensible system of allowances with an hourly rate and introducing a bidding system would help a lot, but it would mean the cake will be cut more equally and some will lose out.