Prices for ice cream vary wildly depending upon where you're buying them. I think that my local burger van is currently charging about £1.10 or £1.20 for a Magnum but I suspect that the seaside stalls in Southwold might be charging double that. (Most of those stalls were charging £1.60 for a can of Coke over a decade ago).
As has been suggested above, buying in bulk is the way to go. Packs of 4 Magnum Classics are currently priced as follows in the big supermarkets:
Waitrose £3.20 or 2 for £5.00
Tesco £2.00 (reduced from their regular price of £3.20)
Sainsbury's £2.00 (reduced from their regular price of £3.00)
Morrison's £2.00
Asda £2.00
Lidl £1.99
Iceland £1.89