In the case of this shopping centre (and others) and some larger shops there are security guards. At the little Sainsbury's I use it'd give him something to do! Tbh I think that shops also ought to give their staff more backing than they evidently do in refusing to serve people who won't mask up, although that's not realistic for small places. Tbh it doesn't need to be rigidly enforced everywhere, but enforcing it more firmly in more places would help to get the message across.
Finally - although it's not enforcement per se - plenty of places have nice, polite signs requesting that people wear masks. If these were more prominent and more firmly worded, as they are on some shops, that might make some difference.