Some years ago when I set off to Italy on my M20 I asked the AA for a quote and the price was ridiculous, but then I found that most historic insurance policy's offer a breakdown service with European cover included, I'm not sure if Brexit has changed that?
A couple of points though, the small print may say that if recovery costs are too great, you may loose the vehicle and be given a payout instead, also some historic insurance doesn't cover you if you have a pillion passenger on board, I suspect you would need to inform them and they would add a small cost to the total.