Re: Seems fine to me
The original owner was a literal victorian plumber, so I'm pretty sure it'll have been wired up by a mate with the best stuff they could get away with not fitting to someone else's house.
We've still got the original victorian mains line coming into the house in the basement, which I'm pretty sure was more of the same since the coating looks good all these years later and I've seen thinner cable used in factories. (same can't be said for what's left of the original fuse board after we got a smart meter fitted and it pretty much rotted off the wall. Mains cable is now holding said meter to the wall due to how overspec it is).