Yes driving instructors are few and far between. Have you looked into a 'driving society' type thing? Or find a driving show. Either should list people who train or be willing to help you get started. Good luck.
Being in the UK, you might look up the British Horse Driving Society, find some local clubs near you to ask questions from. Not sure if the BHDS lists trainers, but they are out there. I do think they are licensed in the UK to be Trainers, have passed testing in knowledge, teaching methods. Have fun!
For future reference, if anyone is ever in the Ocala/Orlando area definitely check out the Florida Carriage Museum. The museum is wonderful and they offer all kinds of driving lessons as well as holding horse shows on their gorgeous facility. I'm lucky my grandparents live so near. It's worth a look even if you're not necessarily into driving.