Calor Gas have announced that they will now continue to refill the small 4.5kg butane and equivalent propane but only as long as the existing fleet of bottles remains serviceable. They will only be available at distribution centers, i.e., not at marinas. You can see the the announcement here.
https://www.calor.co.uk/news-and-views/update-to-cylinder-range (Calor have deleted the link Jan 2024)

We regard this proposal as useless, and our advice remains that if you can source an alternative to Calor that can fit your boat, and you can obtain it, go for it. If you can fit the larger Calor BBQ gas bottle in a locker, this might also be a good solution. We hear reports of boater trying to fit 6kg bottle horizontally into a locker. Don’t do this, as bottles must be upright to operate safely, allowing the liquid gas to evaporate in the bottle. We will consider what further action is needed when the BSS review is complete. We are in touch with some of the coastal user groups who are seriously affected with more than 10,000 boats with gas lockers tailored exactly to Calor’s sizing. Of course, Calor’s decision is commercial and they are not obliged to do anything that they regard a poor business.