Yes, 600V DC irrc. The London tube actually has a fourth (centre) rail. It returns current through this insulated rail, rather than the running rails, to prevent corrosion of surrounding metal structures, pipes, etc.
UK suburban 3rd rail systems are also 600V DC. They used to use mercury arc rectifiers, I don't know about these days.
In some areas (the south east basically) full sized trains on national rail are 700 V d.c. third rail, in most of the other electrified areas it's overhead 25 kV 50 Hz. Then there are lots of areas that only run diesel trains. It's a bit of a mess.
Yeah, I remember when I lived in Southampton for nearly a year, back in 1971.
Some of the level crossings had an attendant full time, who closed a full gate, completely blocking acesss to the crossing when the rails were "hot", not just a boom,like I was used to from Oz.
Other trains into SOTON were diesels.
When I travelled further afield, I discovered a different mix, this time diesels & overhead cable pantograph types.
Years later, back home, when Perth WA was first being electrified, the bus I took to work used to cross the line near the city station.
When the lights went green we waited for a moment till a truck with a big excavator from the work site crossed.
They got their heights wrong, & hit the already activated overhead cables, causing a blinding flash, & eveything shut down.
We were still running diesels, so loss of 25kV didn't automatically shut them down, but it made all the lights red for that section.
Needless to say, I was late for work!