Can anyone advise how long is "long enough" please?
to be on the safe side, especially if it's your first time indirect or through a US airport
if your connection is at a US airport, allow at least 2 hours. Allow 2h30mins for a larger safety margin, and 3hrs if you want to be safe.
Some airports like JFK could require more time, so aim for 3hrs at JFK, MIA, ORD for example
return journey should be ok with 1 hour or more.
with less time you might make your connection, but your luggage might not.
I'm thinking we'd prefer to just use the one airline but what if we changed?
if you book on the same ticket, you're good.
for example Virgin to the US, then Delta to MCO, on the same ticket is ok, you're covered in case you miss your connection, they'll put you on the next available plane
if you make 2 separate bookings (same airline or different airline) then il you miss your connection, you will need to buy a new ticket.
so book only on the same itinerary/ticket (book a London-Orlando ticket, not a London-SomewhereintheUS + a somewhereintheUS-Orlando) (enter london as a departure city, and orlando as an arrival city and you'll get continuous itineraries, be advised that there are 2 international airports in Orlando, MCO is the one with the Magical Express and is quite close to WDW, SFB (sanford) is a long drive away from WDW and does not offer magical express)
Do we have to get the luggage back and go through check in again?
yes on the outbound flight for a US or a CA stopover
procedure is as follow
plane lands, you disembark, go through US immigration, collect bags, go through US customs, drop bags off at the connecting flights counter just outside of customs (takes 2 minutes), go through security, go to your next boarding gate
on the way in, you don't need to collect bags, they're checked trough to the UK (if you're on a single ticket/itinerary, if you're not you collect and recheck and this takes loads of time)