Brit, you keep coming up with more and more outlandish 'what ifs'.
'What if' the sun asploded tomorrow?! 'What if' all the seas suddenly dried up?! ZOMG!
Seriously, what makes you think your instructor is such a goombah that she'd leave a horse and rider in the dust and not realize it? Or that the other students with her wouldn't be aware that one of their number is missing?
You're getting yourself worked up for nothing, because there's nowhere in the UK that you can get lost where civilization isn't within a 10 mile radius.
It's an island
fer cryin' out loud, not a humongous continent like North America. Try getting lost in 10,000 acres of forest, then we'll talk.