Thanks to everyone for their suggestions.
Now if there is someone that would take this challenge on, you can have this nice Harry Quinn frameset for £25 + postage. Aside stripped bottom bracket threads there are also
- couple of dents, one under down tube (non visible unless upside down) and on drive side rear stay
- stuck seatpost binder, which can be cut out easily, but I'm applying Gas Plus regularly before doing that
I think forks are worth more than that, but it would be pity splitting them from the frame.