Thanks for a great set of replies.
I think I'll continue, more or less, as I have before. Filter very carefully (although I've never come across a hostile reaction to my filtering), eyes everywhere, including for doors opening.
Stop if local bikers have stopped, and have a good look at all the cars ahead in case the police are also in the queue.
Stop filtering when close to the cause of the queue.
Take a crossword to do if I do get stuck behind stationary police like last year.

Thanks Steve for the night-riding suggestion. I used to do that but now I really prefer just to ride during the daylight.

To Vaufi and all other German bikers - good luck in getting the law changed.

Here's a supplementary question, prompted by some of the replies above.
I find in the UK these days, most car drivers will pull over to make room for bikes to pass.
But we have, maybe, the greatest density of speed cameras in the world. So after quite a few years of training, I've trained myself to stick to 30mph whenever I can't be bothered to look out for them, or know where they are but think I might accidentally forget one. It's been hard work, but I can now stick to 30 without feeling like I'm locked up in the tiniest possible cell and someone's thrown the key away.
But, I still find the car ahead will pull right over to let me pass, and I don't want to!
I just want to stick at 32mph in case I forget myself, and the next camera, and get a ticket!
How do you signal to the considerate driver in front that you don't want to overtake?

Thanks again for all the replies.

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TTR250 - London to Cape Town