This is just the same as GPS but with a different reference source, surely?
There's very little information given, but my understanding is that this would be a primary rather than a secondary system, i.e. not relying on anything in the aircraft.
As someone with considerable experience in this area, I can see all sorts of potential problems. It seems that the proposal relies on time differences for reflection of signals from multiple TV sites. I'm assuming that they would be looking at the TV signal carriers rather than the modulation on them, but this is complex matter with the current COFDM signals (8,000 carriers per multiplex)..
And each TV channel is used many times over across the country. Re-use of each channel is planned to minimise co-channel interference to domestic reception on the ground, but an aircraft will see and reflect signals from many transmitters on the same frequency. I'm not sure how the proposed system will sort them.
To deal with chevron's points:
1) the ITU would probably take little interest unless some modification was required to the TV transmissions, since the frequency planning criteria would not be affected. And if this was just a UK system, foreign broadcasts would not be needed.
2) and 3) yes absolutely.
I really believe for a whole host of reasons this is a non-starter.