UKC

Our camper is booked in for its MOT soon and it has just developed a fault on the driver's door. Something has broken in the mechanism and although you can get out from inside the van you can't get in the driver's door from the outside. We use the side access to get in instead.

Will this be an MOT fail on safety grounds?

I'm not 100% certain but I think it is.  

Yes. But 10 seconds of googling will have already told you that...

https://www.motester.co.uk/mot-test-of-doors

A very quick Google:
https://www.mot-test.net/mock-test/doors/#:~:text=MOT%20Related&text=Do....

Presumably they can fix it for you while it's there.