I'm sure it is easier to keep them sitting on board. But this situation crops up every year, often twice a year spring/autumn when the fog is around. There must be countless long hauls that have to make this divert and then wait. So LGW mgmt must have well polished plans to serve food and drain the lavvies AT THE LEAST.
What do other carriers do?
That's more down to AI management than the airport themselves. If AI have no service partner contracts (for waste/water, catering etc) then no one is going to bother coming out, until AI make phone calls and confirm payment for such services.