Although there are strict guidelines for MoT testers there is always an element of interpretation. It may be that the effort required to release the seatbelt means it could prevent someone from exiting the car in an emergency.

It may be that some lubrication would be enough to free up the mechanism, so it might be worth trying this before getting it repaired. Main dealer pricing for repairs will always be the most expensive, so we would definitely suggest shopping around for an independent garage. This should be a simple job too, so you may even want to consider purchasing the parts and fitting it yourself.