Why are jacket potatoes becoming such a street food phenomenon here?

Exactly. A cup of tea in Greggs etc costs, what, £2.50? £3? You're literally paying for hot water and a single teabag, they make an absolute fortune on it. And they've done the market research to figure out that their price is an acceptable one, because it's the 'market value'. In that you could walk into any equivalent shop and pay more or less the same amount. They could undoubtedly halve the price and get double the customers but then they're no better off - in fact they're probably *worse* off because they'd have to pay for more packaging for the drinks and more staff wages to serve all those extra customers. Then you've got situations with a 'captive audience' like airports, gig venues etc, where they can charge whatever obscene amount they like because there will always be enough trade from those who absolutely *cannot* go without the drink/snack/whatever to justify keeping the operation going. A JD and coke is about £10 at a gig venue these days and the cost price is probably a tenth of that, maybe even less. (A 1L JD bottle is currently available in Tesco for £25 with a clubcard or £35 without, that's 40 x 25ml shots from one bottle, or 88p per shot, even at the higher price!)