The prices can vary depending on the location. A Mr Whippy van working the suburbs of middle England might charge c£2 but another at a busy event or the one sat outside the Tower of London can easily command more.
I only have the smallest size ice cream so perhaps that's why it was only £2. They're not proper flakes though - much smaller than those you buy in shops.
The prices have always been bumped up at events and outside big tourist attractions because the vendor usually has to pay to pitch on those sites.
Where I live you have to text the ice cream van to call in your street. I don't know if the council has stopped them playing their tunes willy nilly
I remember a van in Hyde park charging £3 for a tiny cone at least 20years ago, lots of upset kids when the parents were saying no, it got quite nasty.