The pipe does appear to be galvanised - it's not rusting anyway. I didn't realise you couldn't weld it. Why not? Can't you just grind off the plating? That's going to be a real pain, as the only other option is to get a threaded section to fit the existing joint, and who knows what thread pitch they've used?
I only measured it roughly with a tape. It's about 90mm OD and 80mm bore, and the thread pitch is about 2.25mm. But, is there any guarantee it's a British type anyway?
The zinc galvanizing gives off some toxic fumes, I don't recall what it is, but welders have told me it's nasty stuff. I don't know if it could be fatal, it can certainly make you ill.
Edited to add; It's called metal fume fever. I didn't know that.
http://en.wikipedia.org/wiki/Metal_fume_fever
You can weld the steel, but then the joint is unprotected, unless you can get it hot-dip galvanized after fabrication.
The screwed joint is very probably a BSP thread, 3" BSP has 2.309mm pitch. It is very likely a 3" pipe.
http://en.wikipedia.org/wiki/ISO_228
Viking Johnson fittings should sort it.
http://www.vikingjohnson.com/pdf/maxi_fa.pdf