Comment by tialaramex

UT1 is what you'd actually use if you specifically want solar time today, yes.

I'm just highlighting that in most places when it says "GMT" it does not mean solar time, just like how "New Street Station" in Birmingham is not in fact a New Station, or on a New Street, it's a station which was close enough to the road in Birmingham named "New Street" that it's forever associated.

When actual Grenwich Mean Time was "still a thing" astronomers would use it for solar time because we had no better approximations. Today the IERS looks at very distant objects in the sky. We know that in some sense (relative to us) these are not moving because they're incredibly far away, and so we can use them to measure rotation. And then we adjust for where the Sun is from that data.