Yes, Google Earth is probably best. The lat/long in Google Earth is WGS84 and its georeferencing is accurate at the few metres level over much of the UK (mainly the flat bits since projection on mountains is much harder). A few years ago I checked this for my telescope using a surveying SBAS/GNSS receiver which, ultimately, is cm level accurate in WGS84 if you leave it long enough. The position it got was around 1.5m from the position I read from GE and about 6m different in height although I couldn’t work out which geoid reference it was using.
For most astronomical purposes, apart from very precise astrometry of near objects and meteor triangulation, you don’t need that kind of accuracy. In any case, at the metre level you need to be very careful what system you use. Lat/Long is generally WGS84 but height depends on the exact reference geoid and, for instance, the GMN meteor camera network uses EGM96. I think the difference between those two can be 10s of metres (height) in some places.