Ben - It doesn't send SMS messages at all and no SIM card. The SMS is just what your contacts receive. You set up fixed messages on your account beforehand. It then uses the satellite to notifiy position and message type to ground station to pass on to contacts.
Polwart - The message handling company are GEOS - see www.geosalliance.com
for more info. They don't just do SPOT, but a lot of corporate activity as well, but yes, they could go bust in theory.
They are legal for use in UK - I don't think they would be available from the distributor otherwise.
It's just the 'OK and 'HELP' (like a PAN-PAN really) messages that get sent directly to your contacts. For a 911 message, they will validate location with your contacts and then contact authorities. Think of it like an alarm monitoring company calling the police for you if there's a break-in.
If you take the tracking option, it could be quite a useful tool for non-emergency use as well eg watching boat delivery progress.