If anything ever actually happens as a result of the announcement, and some regulations are actually changed, it's going to be utterly trivial and have very little effect -- manufacturers aren't going to redesign their packaging, and certainly aren't going to change the size/volume of their products without having very good commercial reasons, and now that we've had metric measures for decades, even people like me who grew up with imperial measures (and £sd) are now accustomed to thinking in metric.
In practical terms, it makes very little difference, I think. I know how much of something/how many I want to buy, and it makes every little difference whether I'm buying 454 g of sausages (i.e. 1lb) or 400 g, or 410 g, or various other weights offered by my local supermarket. What matters is whether I'm buying 6 or 8 of them, and what size they are.
It's just Johnson desperately trying to look as if he's doing something, possibly because of this
though heaven knows why they think it's going to make much difference.