Can't help with the UK specific questions as we live in Belgium - we have an 8 year old and a 3 year old who have been been basically living in a van for 2 and half months each summer since they were 5 and 1:
We have 3 point seat-belts fitted and for our youngest use a britax and mow a michelin (europe) seat our eldest has progresses through fiited seat and now a brtax adujstable back booster.
Whe we just potter around locally (within a 200km radius) we have taken a large britax buggy (pain fro camping but day to day ok) a small mother care stroller great for camping and even out Charriot cheetah system which is a bike trailer and buggy/stroller combined Chariot USA
But for camping a backpack while they are under 2 is even better
As for sleeping our eldest uses the poptop he has loads of room and our youngest a cab hammock that we got in germany but just campers and bluebird sell them in uK. All we did was rig up a cargo net just in case
They have both done over 20,000 k in the van and like i said that has included 2 x 40 day round europe wanders