I'm with 3 and I made a trip abroad 2 weeks after joining, so I rang to find out what the data costs would be (as I hadn't been a customer for 30 days as the small print says you must be),
But "Oh no sir, it works straight away from the moment you join" so it may just be advertised as 30 days to discourage 'one off holiday purchases', you can also drop to the cheaper tariff after your trip.
I also had a weeks skiing in Meribel (France), the signal was reasonable in the resorts and a bit patchy higher up, might be better nearer roads of course