Hi
@MissMagic and welcome to the forum! Nice to have new people join us!
There have been no simulated sailings yet here in the US, so I don't have any hard data to answer your questions. And if DCL is going to do a simulated sailing in the UK, I have no idea if the British gov't will impose its own rules on the cruise line. So the best I can do is give you my speculation.
Crew members have been reporting to the Magic over the last two weeks, but I don't know if the ship is fully staffed yet. I would guess that they'll need all departments to fully staffed in order to truly simulate a real cruise. So once the ship is fully staffed, I supposed they could sail; and of course, it would have to be before the Staycation cruises begin.
You wouldn't. It will not be open to the general public for normal booking. They may ask certain groups to be volunteers, but they won't be opening up "booking windows".
So we had a conversation about this question a few months ago here on the forum. But our guesses were based on the assumption that any simulated sailings would originate from the USA. So we speculated that the passengers would be DCL employees, families of crew members, and employees of other parts of the Disney company. I imagine that most of that group of people would not be available to fly to the UK for a 3- or 4- night sailing. (Or more likely, DCL wouldn't want to pay for them all to fly to London for the cruise.) So I'm not sure who would be asked to part of a test sailing. Maybe they would solicit volunteers from Castaway Club members in the UK, but I honestly don't know.