I resisted loyalty cards for a long time, but recently all the large UK supermarkets have started offering loyalty-only pricing - so not using a loyalty card ad

I resisted loyalty cards for a long time, but recently all the large UK supermarkets have started offering loyalty-only pricing - so not using a loyalty card adds maybe 5-10% to your bill. They all profile on online shopping and debit/credit cards anyway, so I figure they don't know much extra about me.