the Airport Operator bases the decision on the TAF, and the TAF is based on the computer met model, so in practice, it is all down to what the computer says.
Which met office runs the computer model? Recently I bumped into the ex CEO of the UK Met Office and he talked about how he transformed the role of his forecasters (a word he despises because it implies their job ends for the day after issuing that bit of paper) to become more effective by placing them inside the control centres of their customers to help them develop better decision/knowledge systems in relation to the meteorological information that is available.
Seems to have been hugely successful, the only problem is that there aren't many of these meteorologists to spread around the place so the benefit needs to be substantial to UK interests to get a buy in.