Square Leg field exercise- Part of the exercise involved a mock nuclear attack on the British Isles. It was assumed that 131 nuclear weapons would fall on Britain with a total yield of 205 megatons (69 ground burst; 62 air burst)with yields of 500kt to 3mt
Mortality was estimated at 29 million (53 percent of the population); serious injuries at 7 million (12 percent); short-term survivors at 19 million (35 percent).
Square Leg was criticised for a number of reasons: the weapons used were exclusively in the high-yield megaton range—with an average of 1.5 megatons per bomb—whereas a realistic attack based on known Soviet capabilities would have seen mixed weapons yields, including many missile-based warheads in the low hundred kiloton range.
And also found;
The 2010 Strategic Defence and Security Review set out that in the future each submarine would be armed with 8 missiles and a maximum of 40 warheads.
Most of the warheads have a yield of 80–100 kilotons, although some missiles may be armed with a single low-yield nuclear warhead of 10–15 kilotons. (In fact some are as low as 0.5-1.5 kt if they choose not to activate the internal booster)
So like I've been saying, these 25/50mt beasts are tests, they are too impractical and expensive to build and maintain. Nearly all arsenals are made up of low-yield types these days. There might be the odd big one about but nobody's going to waste it on little old us...
