I don't think so, although most of the old Great Western (ex-broad gauge) would probably be upgraded quite easily.

But the advantage of using the Great Central is that, even tho it was shut by Beeching, large amounts of the basic infrastructure is still in place and it was built close to UIC standard.

however, the section south of London is mostly under-used and so renovation work wouldn't be too much of an inconvenience.

keep to the Fen Causeway