It depends, do you really know , what the colony does when it flyes in rain! When I look British forecasts, your temperatures are so low, that flowers do not make nectar. And if nectar is wet, bees do not forage it.
What bees do in bad weather, they bring water to make food juice to larvi.
When I compare your writings about your busy bees and annual yield, there is something odd in these stories. My experiences say, that you have out there so much colonies per pastures, that flowers cannot give food to such amount of bees.
I just took away the crop from one Italian hive, perhaps crossed with buckfast, and there was 80 kg capped honey. Most of honey must be aphid products from trees. Hot summer killed the flowers.
According you busy bees raports, you should get 150 kg - 300 kg honey per hive. They gather even in December yield in low 6C temperatures. My bees get honey over 20C temps.
I have kept German black bees 30 years. I know what they do. Their biggest problem is that they swarm, and a swarm cannot forage much honey. Carniolan has as bad swarming problem. There are Italian varietes too, which have 100% swarming. If you do not select them, and do not renew your bees, so will happen to all bee breeds.
I have tried Buckfasts, but difficulty is that, buckfast genes vanished in 3 years. After two open mating they are not any more Buckfast.
They are cross bred what ever. Buckfast hive is too tall to me to nurse, even if it makes huge yields.
That I have leaned in 60 years beekeeping, that my own bees are the best in the world. It is same with your home grown potatoes. Nothing is so good as my own potatoes.