Yes it is. Given the fact when I shop at Aldi, I take my own bags and then use them to carry my shopping (I don't fancy paying £1 for a trolley!), before unloading at the till and repacking, I'd be rather upset if I was jumped on and accused of shoplifting!
I hope you realise you get the £1 back
** Unless this is another way of conning the public out of their money a la Ryanair?