I always look at film prices etc when out shopping. This bargain is so good I have to pass it on. It was in my local
These packs of three started at seven pounds (well, an estate agent's version of £7) and have reached what looks like rock bottom but when they went through the check-out were only £1-99 and the 400ASA/ISO ones started at about a fiver each and the check-out price was 98p !
By the way, Boots slide film appears to have been discontinued. Or rather, I bought some at a fiver reduced from about thirteen quid, tried it and went back for more to find it at a pound a box of 36 exposures. That included processing and, when I bought my batch, they were doing a buy 3 for the price of 2 promotion on film. So I got them at 67p a box.
So I'll wish you luck in the search for it. Two branches near me have already run out of it.