What would you do in this situation?

A lot of my riding is along country roads, which are single track. Just wide enough for a car plus bike if they are side by side and respective tyres are in the gravel/mud at the side of the road.
Most oncoming cars realise it’s a squeeze and slow or kindly stop. Some, however, do not. I’ve had situations where I’ve been passed within inches at 25+mph.
Q - how do you stop this happening?
It arises quickly. There are about 2 seconds between realising they won’t slow, and it being too late to do anything. I can’t slow to a crawl each time a car approaches, it’s not practical and would mean I can’t cycle above 7-8mph. I do slow significantly on each occasion and stop when conditions dictate, of course. I have good road sense and know about dominant positions and negating bad driving by not giving room for drivers to enter. I’m also courteous and thank each road user for wide overtakes and consideration shown.
My instinct would be to take a more dominant position near the centre of the road so that the oncoming vehicle cannot pass and is forced to slow. On a practical level, you simply can’t do that. You can’t pull into the path of someone doing 25-30 mph towards you, hoping that they will stop in time. If they think they can get past, they simply go for it. There will only be one loser there if they don’t. There is no alternative but to try and brake as quickly as possible and hug the loose grip gravel at the side. This can be scary - in the recent storms, your line can deviate, and it was extremely close one time last week.
As an aside, the drivers to look out for are the old (70+) ones, in this situation. By far the most dangerous I’ve found, not giving an inch or slowing. I guess they can’t judge distances and are not used to bikes, slightly ironic.