Checking Amazon UK[0], it seems there are 18g, 39.4 g and 51 g Mars bars (and 57 g Mars Protein bars). So it's not clear weather a 60 g Mars bar is that much of a good example of shrinkflation, but 40 g seems to be the wrong comparison.

[0] https://www.amazon.co.uk/s?k=mars+bar

Wikipedia's page claims the "regular" was 62.5g and now 40g https://en.wikipedia.org/wiki/Mars_bar#Sizes