How do I identify a 6v electrical system on an older British car?

Don't know. When our family had a '54 Jag XK-120, my recollection says that it was 12V, don't remember positive or negative ground. Seems to me it used two small six volt batteries in series, back behind the seat. Been a long time.

"Which ground" is important. If you aren't sure find out before you hook up a new battery. One way to tell would be to turn EVERYTHING off you can think of, except the headlights. Unhook the "B" (bat) wire at the regulator for safety until you find out. With the headlights on, hook up the battery "you think" the correct polarity, BUT wire a stop lamp bulb with clip leads IN SERIES with one battery cable. This will limit current in case of a SNAFU. Then look at the ammeter and make sure it moves to "D" (discharge, to the left)

Frankly, before I'd touch that thing, I'd be looking for "the book."