Well, sometimes you just have to pay with what you've got to hand! A bit like buying a newspaper and only having a £20 note on you.
Why did a queue build up anyway? It's not like it takes more than 20 or 30 seconds to count out 58 identical coins - it would take just as long as that to pay by credit or debit card. You do get some cashiers who are like "one... two..." etc, but that's hardly the fault of the person paying.
If supermarkets had machines where you could put in twenty £1 coins and get a £20 note out (and I'm NOT talking about those rip-off Coinstar jobbies that take a massive cut of whatever you put in and only give out vouchers!) then I'd agree that handing over a massive handful of £1 coins would be inconsiderate. But they don't, so it isn't - as I said, sometimes you just have to pay with whatever you've got to hand.
The worst one is when someone has about a million vouchers and coupons saved up and they decide to use the whole lot of them on a single shopping trip. Takes absolutely ages to put them all through.
By the way supermarkets don't HAVE to accept any coins, cards, milk tokens, notes or anything - in fact they don't have to serve you at all if they don't want to. Many shops already refuse £50 notes (and £100 notes in Scotland) due to the risk of forgery. The "legal tender" thing, where you're limited in terms of what coins you can pay with, applies only to the settlement of debts.