Yes, I'm local to you :) - I will pm you a localish Facebook group that you might want to join, lots with Hypermobile kids on there, so a good place for advice for you
It is scary stuff when you first start looking into it, but it differs for everyone & lots of kids grow out of the worst of it post puberty - it can get worse on the run up to puberty though, which has happened with my own DD, but knowing what it is, means they can be helped & therefore avoid problems when older, so not all bad. The POTs symptom I've read us uncommon in kids, often triggered by trauma, pneumonia at 5 in my DDs case - but I've read that when the get it young, they grow out of that post puberty too
We've avoided a lot of car travel, go by train where possible TBH, but sometimes it's necessary - things that have helped DD have been...
the travel sickness accupressure wrist bands - you will find these in boots & they come in kids sizes too
Distraction such as music to listen too on headphones - reading, video games etc will make it worse, but we found a portable DVD player helped when DD was small
Ginger sweets to suck - infinity foods have them, they are in a yellow box & have a yellow old fashion wrapper - sorry I forget the name though
No air freshener - my dad always had these in his car, the real strong chemical ones & DD was violently sick every time she got into his car
You can use lavender oil or rosemary aromatherapy oil as air freshener instead & they help with nausea
Small bucket & lots of bags to line it in the car, along with baby wipes & kitchen roll - DD used to help herself to these & became really adept at sorting herself out from a young age - she preferred to do this, she was embarrassed by it.
Avoid milky drinks, milky foods etc fir 1,1/2 hours before travelling - this made a huge difference to how often DD was actually sick - she still felt ill, but didn't vomit
Avoid bumpy or windy roads where possible - especially roundabouts
As soon as tall enough - let them sit in the front seat - this helped DD a lot
HTH