We do
We are when we need to be, but the UK's needs are very different to France, Germany, South Korea, Japan or China
Most of whom went bankrupt very quickly.
Why build a totally inappropriate standard of railway simply because other, very different, countries are building HSR? Look at HS1, HS2, Crossrail and the NPR project for what the UK does when it needs to be radical. But this would be akin to building a motorway to a village. It's. Just. Not. Necessary.
And when it does we will be ready for it. Doing a project in phases isn't a bad plan. This line already has trains running on it, so to upgrade it to your specs we'd need to cancel all the freight traffic while we do so. In the UK, this will probably result in the freight never coming back to the railway.