bcnboy - London airports may be restricted, but transport even by high speed train from London to the Severn will take an absolute minimum of an hour, even if HS3 is built. Those with money who buy the expensive air tickets will prefer to fly from an airport near London, leaving just the cheap tickets being sold from Severn airport. The corollary to this, is that Severn International wouldn't really attract much in the way of premium airlines. The result would be the UK having paid tens of billions for an airport and a high speed rail line, but nowhere near enough passengers willing to pay to use it.
If the UK is going to spend serious cash to expand air infrastructure, the only place to do it is near London. There just isn't enough cold hard cash sloshing around anywhere else in the UK