Network Rail (who own and maintain the track/infrastructure) are state run.
It was privatised for about 5 years, it was a disaster, and it was brought back in house.
Trains are run by TOCs, though these are being gradually coming back to the state too.
The honest answer is, we have a fuckton of track, much of which has been there since victorian times.
Couple that with diesel-electric trains that run at 125mph already, a lot of track that doesn't get that much use, and the electrification number is low.
We're slowly getting there.
New lines are electric from the start, and electrification projects are rolling out.
It can be a pain in the ass though (GWML, for example. We had to order bi-mode trains to continue down to the westcountry, and while the electric part was completed.)
Plus the issue that any track that's busy enough to prioritise electrification on is going to be more complex because of the impact any downtime causes on a busy route.
I once changed a friend's autocorrect from "regards" to "lots of love".
And I somehow managed to keep my mouth shut long enough for a lecturer to get a very funny email.