I've not encountered big stores (in the UK) charging for their loos before but, if they're going to do so, 10p seems really cheap to me.
It's 30p or 40p to use a loo at many of London's mainline rail terminals or 50p in some of the public toilets in Westminster. (I don't know whether they still charge on Scarborough seafront but it used to be 20p over thirty years ago!)
Some pubs now permit non-customers to use their loos but only on payment of £1. A decade or so ago there was a filling station on the A12 which also charged a quid to use their loo - even if you'd just filled the tank of your vehicle with fuel from their pumps and bought loads of stuff in their shop!