If I fly to dive it takes an extra day off work, that is expensive.
The whole experience in the U.K. is better. You can drink the water, flush loo paper, eat food which probably isn’t designed just for British tourists, go outside in the sun, know how the parking works, have rescue services that work, know that safety regulations might actually be applied and if you hurt yourself you will get treated without a credit card.
And you will not find yourself diving with numpties unless you brought your own. Darwinian pressure and all that
It used to be that you were not supporting some evil authoritarian regime either
And you are not trashing the planet to get there for a week.
These days you can book a B&B through the like of booking.com which allows for last minute cancellation if the weather is obviously hopeless. Or you can go anyway and just enjoy the place with a bit of wind. Cornwall, Permbokeshire, Orkney, the West Coast, Shetland, Portland, Swanage - these are places full of people on holiday anyway. Not so bad really.
Traffic in the South West is tedious. Our club usually does bank holiday weekends and it can be very bad. However when I came back from St. Kilda last year the only problem I had was someone having an off on a narrow road which was then impassable due to ambulances etc and so there was a 50 mile detour. The bit between Glasgow and London I averaged 68 mph (excluding wee breaks).
While it might only take 4 hours to fly to a warm place, you still have to get to the airport, check in and hang about for a couple of hours, pack all your kit into airline resistant bags, haul it about airports, in and out of cabs or buses and generally hope it arrives safe. It takes all day or worse.
Unless you are going to the islands you get anywhere in a day in the UK.
And all that before you get onto the various achievements of U boats in two wars, a long coastline facing the Atlantic and the odd bit where the mega fishing boats are kept out. Oh, and the results of letting alcohol near boats.
For localish stuff I try to overbook. Some day she will be blown out. You can guess a few days ahead but usually only really know the night before. So on average you get the diving you want, just with a few spare days left. Brighton Marina is an hour and a half for me, so an early morning start and a relaxed afternoon. At the wrong time of year the vis might be poor and you have to concentrate on the small stuff. But there is small stuff to see, unlike some places.
Did I mention St Kilda? It is absolutely brilliant. But like U.K. diving in general it is an effort. You can’t just ring a travel agent.