The recognition is via the camera on the windscreen. Windscreens can get dirty, streaks left behind, etc. and this can affect the speed sign recognition.
I've heard that you can just clean the area in front of the camera on the windscreen carefully and restore performance. The camera is small, so a small piece of dirt can affect a large area of the camera's vision. It may be that RainX or similar (supposed to repel water beads) could help. I've never had the problem myself, but here in sunny Queensland, we don't get road salt or many days with rain.
In fact, we're headed into a El Nino season, and our problem is that our water tanks run empty. Can I get a bit of sympathy from you Brits with all your regular rain?
(Ducks)