I’m curious if this is more likely to get lifted off by strong wind.
It’s in the UK, there will be a layer of roofing felt underneath the slate that prevents pests getting in. Roofing felt is the same stuff that shingles are made from in the US but comes in a roll. So in the UK we have basically the roof you’d have in the US and then another slate roof on top, it rains a lot here, we probably have some of the best roofing in the world.
Nah mate, roofing felt or tar paper is an underlayment for asphalt shingles. While roofing felt uses similar materials as asphalt shingles, we use both underlayment, and shingles (sometimes terra Cotta tiles in the southwest)