There will be some kind of agreement - even if just only temporary - that EU and UK carriers are allowed to continue to do business as usual.
For UK to EU flights or vice versa it will still be possible (I would assume without permissions even).
For intra UK flights there might be some kind of carbotage rules in force but don't overestimate the potential of intra UK ops to EU carriers at the moment.
Overall the UK carriers have much more to loose than EU carriers if it comes to commercial business charter OPS. It will be - in the future - difficult to operate intra EU flights for UK AOC holders and the market is much bigger than any intra UK market.
Additional to that the UK charter scene is almost non existent anyways for the past couple of years. Whenever you land at the large UK Bizav airports like EGLF, EGKB, EGGW, etc the majority of carriers are based outside of the UK. Funny for the largest market in European aviation, isn't? Tough times ahead for the remaining commercial Bizav companies left in UK.