You can buy some low priced items such as produce, and stealth away the higher priced goods into a personal bag while shopping.
It creates less suspicion than just walking straight out if you clearly have stayed in the store for a while.
I'm in the US so not sure of the exact differences, but based on what you've described, my method would probably be to bring two reusable bags. One gets put in my basket or cart, and filled up with stuff as I'm shopping around the store, then slung over my shoulder (key is to be not too greedy so the bag isn't too heavy/deformed, only fill it up 80% or something) as I go up to the register and buy a few cheap items. The second reusable bag gets handed to the cashier for the couple cheap items I buy, so nothing looks out of the ordinary. Just looks like I have an extra reusable bag that I didn't end up needing for my groceries.