Long term I would be surprised if the UK government doesn’t stick to something close to the 80/20 rule even if they have the ability to change it. It’s long been government policy to encourage greater competition and to maximise the use of the runways capacity. You can’t do that without a usage rule. The only tweak I wonder if you might see at either a UK or EU level is something to stop/discourage slot sitter flights because of the environmental impact.