Why 225 mph? Why not 500+ in straight lines ie point to point?
I mean if we really need super high speed trains (and all the infrastructure and power etc to operate them then why not make it worth while?
Besides, the island of Britain is barely a thousand miles end to end. Being able to travel less than a third of that in an hour can already be done easily, hell, you can do Lundun to Mancchester in a car in less than five hours without having to sit next to anyone...
Your supposition that it is the most cost effective solution is misguided because the actual most cost effective solution is not to bother building it in the first place
Because even the World's fastest commercially in service train, not as a test run, the Shanghai Maglev, runs at 'just' 267 mph, and only over 19 miles. It reflects that technology has not YET provided the speeds you suggest, and as you say it would need a straight line, without too many curves to allow such trains to run on. Now, in the UK we are, as you rightly say DG, a small island, and certainly not like the expanse of say the USA or Australia.
The UK has major towns and cities quite close together, and certainly not in straight lines if you try to link them up with a HS railway. The cost, once more, would also be horrific if you tried to build very straight lines as, inevitably, they would plough through areas of habitation requiring the mass demolition of property. The HS lines have, and are, causing much concern on that score, even though the planners are attempting to bend their new lines so as few properties are effected.
Yes, you are right, the UK could stop building new railway lines. But then we are capping any possibility of UK Ltd., developing it's business in the Midlands and North, and letting the current railway system, let alone road system, be completely over loaded and grid logged with all the additional population this country will have by 2050 and beyond. It is already being predicted that by 2045 the population will reach 76 million.
We could just sit back and watch our proud country become a poor, backward thinking, over-crowded backwater of Europe that has given up on any desire to remain a leading World economy. Do we all really want that?
So many people are concentrating on the multi-billion pound cost of building these lines, whilst ignoring the trillions of pounds they potentially will generate over many decades by expanding the business potential of the Midlands and North, whilst providing a very fast, highly efficient and environmentally sympathetic transport system for the rest of the 21st century.