Why is UTC activation time referred to 30W in NAT-OTS?
Think of 30W as a kind of traffic marker for flight planning.
If your planned route is going to cross 30W before the tracks activate (that is, ahead of the traffic flow), then you can file "random route." If you're crossing 30W during the active period, then you have to file on one of the tracks unless you can climb above them before crossing the entry fix.
This also works going against the flow: if you're crossing 30W in the opposite direction during the active period, you'll have to route either north or south of the tracks unless you can get above the NAT FL's before your entry fix.
As far as Zulu goes, it's just the common reference everyone uses (therefore, "Universal Coordinated Time").