Usual advice is that chillies get hotter if stressed, ie, regularly allowed to get to looking sad and drooping before watering. The BBC trial appeared to confirm that: their chillies planted in the ground and not regularly watered were much hotter than the same varieties container grown in a greenhouse and regularly watered. My experience, with chillies container grown but placed out of doors against a hot sunny wall, is that my chillies were hotter early season when they were frequently thirsty than later season when nature reduced their water demand.

I find it hard to believe that a UK-grown chilli is really the hottest in the world, probably just the hottest one that has been measured. The "Dorset" Naga is, it is generally perceived, derived from some seeds taken from some Naga Morich chillis and grown in Bournemouth, ie a selection of Naga Morich rather than a separate variety. The Naga Morich is from Bangladesh, and is frequently on sale in Bangla groceries, though they often don't realise what they have. These groceries may be a cheaper source of seed/fruit than the Bournemouth people. Banglas use the Naga Morich in their cooking by introducing it very briefly to the food. Frankly I have no interest in growing something as dangerously inedible as that.

In general, varieties of chilli bred for their decorative effect, (Twilight, Marbles, Pretty in Purple, etc), are less tasty than those bred for culinary advantage. Most chilli varieties, including sweet peppers, are Capsicum annuum, and there is a range of flavours, eg the mirasols I grew this year have a distinct strawberry-like taste. Habaneros, birds-eye chillis and scotch bonnets are C. chinense, though like all chillis they are from the Americas not China. There are all sorts of other interesting chillis of culinary interest, such as rocotos/locotos (C. pubescens, easier to over-winter than other chillis, though possibly not so easy to germinate), ají-type chillis (C. baccatum, commonly used in Chile, Peru), etc, etc.