Iain,
Yes, Zulu [Z] is *exactly* the same as GMT/UTC/UCT, none of which make allowances for vernal equinox addition/subtractions... Frankly, I think UTC/UCT is just the French trying to poach GMT from us again...
Z makes no differentiation between winter and summer time. As with all such things, it would be easier for everyone if we put down the times in two or three primary timezones (which I will try to make sure happens for the next online event): Z/GMT/UTC, BST and CET.
It should be noted that when I write BST above, I don't mean British
Summer Time, but British
Standard Time; UK Local time (with the vernal equinox addition/subtraction included, so Z+1 right now...)
And perhaps it should also be noted that any political party that wants to manifesto the UK going to Z year round (or even the UK joining CET) and abandoning the poxy vernal equinox addition/subtraction would get my vote, and I'd put up posters and help with campaigning...