Doesn't matter if you're in your 1st year or your 45th, you can refuse them access anyway.
AFAIK, that's always been the case, yes - including staff with footplate passes - except, I presume, locomotive inspectors. When holding a footplate pass, as I remember, it was always understood that the driver could refuse permission to enter the cab, and personally, I always showed my pass and requested respectfully to climb on board. Permission never refused, but it had to be requested. Quite right too, IMO.
And no, I don't think MPs or ministers should be allowed on the footplate without an inspector present. Some (many?) of these people are clueless about the responsibility of train crew, and the concentration and focus needed when on the road, and an inspector should be present to protect any driver from such people.
Coming back to the OP, when I was on BR (W) back in the 70s, we were informed that a Japanese govt minister (I think it was, a Japanese very big wig in any case) was to coming to S Wales on some visit related to investment, and could we select a locomotive that was unlikely to fail for his train. It's a long time ago, but I think I chose 47507. We had it especially monitored on a very early up Swansea - PAD train to check it out. And it didn't fail on the day it was needed either. Well, truth be told, I can't actually remember, but I'm sure that if it had, I'd have got a rocket that I would have remembered.