It probably depends on what area of a particular country you are in as well (larger countries at least, where each state, province, etc. could have different standards). Here in New Brunswick, Canada, the rules say visor and/or approved safety glasses, which is a little different than Ontario.
http://www.gov.ns.ca/enla/healthandsafety/docs/ForestProfessional.pdf
That covers the Atlantic provinces and covers alot more than just chainsaw use as well, but it does give an idea of the safety regulations in this are of the world. It doesn't cover arboriculture work (in the tree), but the laws here say that if a chainsaw is used in the tree, the same safety equipment must be worn (boots, pants, hearing, eye, hat).
My uncle for one only wears jeans and sneakers (or boots, depending on weather and terrain) when he's cutting. He's been extremely lucky (and we keep remind him of that), in that he's never cut himself in 40 years. I keep telling him that it only has to happen once and that's it. He won't listen.. and how do you force a 60 year old man to wear proper safety equipment? For me, I cut myself when I was about 19 or so, about 2 inches below the knee. Somehow, the saw missed cutting through bone and I ended up with a 6 inch scar that stretches from one side of the leg to the other. It was a bad way to learn that lesson, but it could have been a lot worse.
Sorry to get off topic there for a bit, but I do know that we seem to borrow alot of safety standards from Europe, so chances are, we're not that far off from you in England.
Mark