Note that this is an active change to the status quo. Up to and including today I had no need for my UK passport when entering the UK on my Swedish passport. From tomorrow I cannot do that and there's no reasonable explanation given for why this must change.

Edit: And while obviously this is not a big deal for me financially, there are a bunch of pensioners in the 3 million Brits living abroad and for many of them the £100 fee is a significant and unnecessary outlay.