Old manhole cover rating

That's not a BS number, they're usually 3 or 4 digits.

Old grey cast iron is usually non-load bearing because it is brittle, generally only pedestrian use. They've bust it, so it's probably this stuff. The modern modern stuff you see on roads is ductile/spheroidal graphite cast iron which has only been around since about 1949 ISTR.

The cheapest galvanised ones from B&Q are 2T rated, IE rated to carry 2 tonnes, more than enough for a domestic drive

True. I replaced some paved-over hidden cast iron ones after my car went through one of them. Darn, I said. I've since had a loaded skip lorry drive over the replacements (galvanized tray with inset paviors & plastic frame) with no ill- effects.

The only BS for manhole covers I know of was BS 497 Part1: 1976.

I bought 2x 2tonne rated ones to put in my drive and they buckled just with the weight of my car. The local supplier had recommended them as suitable, so I had him come round to inspect them. He agreed to swap both for 4tonne rated ones which have been fine.

I'm not sure how the rating is calculated, but I suspect it is for a distributed load rating rather than a point loading as you might get with a car tyre in the middle of it.

I bought one of these from a builder's merchant to replace a broken 1909 cast iron one on a footpath. When you stepped on it, there was quite a ping-pong as it deformed and sprung back. Changed it for a cast iron vehicle class one (7 or 10 tonnes, or something like that). Was only a few pounds more expensive, but that was before the price of steel rocketed.