APD
I'm not sure it's journalistic hype entirely.
If it were to happen and CWL had a nil apd and English airports had a £12 per short haul apd (it could well be more than that if rumours are true) then it would be a very tempting prospect for, say, Ryanair and/or easyJet if they could charge £12 less per each departure fare with little financial loss to themselves.
In the Severnside scenario the respective catchment sizes would have a bearing as well, and because of the imbalance in numbers and make-up (business and leisure) more people would have to be tempted from the English side to travel from CWL than currently happens in reverse, though numbers of Wales emanating or destined passengers using BRS are substantial – around 750,000 a year according to the last CAA survey in 2008.
The owners of those English airports that would be most seriously affected would certainly have a lot to say if they perceived the value of their investment decreasing owing to government action not evenly spread across all UK airports - this press report may be the opening shot from the BRS management.