I lived in the US for 4 years. Because I was staying there, I had to get a US license & a car (of course!). So, I got this new car - manual, naturally - with the steering wheel and gear stick on the wrong side. And went down to take my test.
I had already passed the TEN multiple choice randomly selected question theory test. Even though the questions were different for everyone in the room, I did get the impression that some people were trying to copy. I can't think why - the questions were somewhat simple - what do you do if you miss your turning on the freeway? a) Reverse. b) Turn round....
So, I'm at the test station with a new car which I had been driving for 24 hours, with the steering wheel on the wrong side, pretty nervous. And in comes the examiner - complete with holstered pistol.

Now, I'm no longer nervous; but petrified. I mean, what happens if you don't pass - do they shoot you?
The test lasted no more than 15 minutes. I recall, I made three or four driving errors. But this was obviously accepable in Boston because I passed.
So good luck with your test. And remember, because this is the UK, the instructor won't be carrying a gun. And the worst that can happen is that you have to take the test again (but I'm sure that you'll pass).