Great place the UK.
If the rules don't specifically say you can do it - then obviously you cann't!
Not getting at anyone but it is interesting the mindset that we seem to have. Try Victoria harbour in British Columbia - rather busier than the Thames has been for the last 100 years. With scheduled flights turning finals over the top of the parliament building. and landing in the main channel of the harbour.
But you're right it'll never happen here.
I think there were all sorts of 'special arrangements' worked out for that visit, although given that Thames are pretty good with transists I would have hoped it was easy enough.
as to commercial viability - I would guess that depends on where all those City bonuses are being spent - surely not N.Wales and the Peak District?