Robin regarding Flight priorities we do not delay flights which may or may not be on a slot for flights with a lower category unless safety will be compromised.
Have a look at MATS part 1 Section 1 chapter 4 page 5.
I'm not to sure where the problem lay with the flights you say were forced into non compliance with their licence, it is however the holder of the licences responsibility to conform to the privileges of their licence, I am not privy nor an I able to ask what qualifications the pilot may have we have to be told and as I said we may not be aware of any problems with visibility in a specific area. Once told of any difficulties I know the colleagues i work with and myself do everything we can to try to sort alternatives.