Strategic Road Network: £700M / 4,436 miles = £98000/km
Not included: cost of pollution, injuries and deaths on the roads.
Not considered: potential and actual capacity of the roads vs the railways, potential for historic underinvestment meaning higher current costs, whether these statistics mean a single track/road lane or all the tracks/lanes, usage costs for passengers and freight.
A breakdown of the railways costs would be useful for a better comparison, i.e. major lines only.
https://www.networkrail.co.uk/wp-content/uploads/2021/07/Ann...
https://assets.publishing.service.gov.uk/government/uploads/...
I love trains. I used them all the time when I lived in the U.K. and in Japan. I've ridden Amtrak from New York to San Francisco, and the Orient Express. They are, however, very much not a one-and-done investment as is being suggested elsewhere in this thread.
People are largely familiar with road maintenance, but most of them have no idea at all about maintenance of the railway.
Maintenance costs for major roads in the UK (trunk motorways and A roads) is something about half that.