Shed
You haven't quite got BA's Reward Saver right. On a trip via London you don't pay any extra for the MAN-LHR sector. So the cost of a trip MAN-LHR/LGW-ALC is £17.50 plus 7500 Avios, each way. BRU and CDG are £17.50 plus 4500 Avios each way.
I fully agree with you, however, about the downside of connecting through LHR, or having to do the LHR-LGW shuffle. Much better to fly direct.