You're correct, it's a way to disaster! At that speed, you've no time to react if a car decides to change Lane without looking (or even drifts in Lane and narrows a gap). Go with where you're comfortable, generally by the time traffic is up to 30mph max, I've stopped filtering and slotted in.
UK car drivers just aren't aware enough, for the speeds you describe to be safe. France, on the other hand, especially around Paris, they are much more aware of bikes, are alert for them, and a lot of the French bikers will hurtle down the middle (I don't think you can call it filtering, at these speeds) at 90mph.I followed a few of them, but could only concentrate enough for a few minutes at a time - it's absolutely bonkers :-)