Not going to be easy to weld is it, I'd be looking at a custom frame builder to handle that. I weld, but me and my MIG are not going to be able to handle that, sorry.
How old is the frame?
That looks like a complete failure right on a weld point too.
Ouch, Is that frame just dirty or really old and rusty? Like Stu said the break on the edge of the lug will be tough to repair, probably will need a new tube which means a custom frame builder. Is it worth it?
Might be time to salvage the useable parts and move on.
Did that happen suddenly or propagate over a period of time?
Judging how clean the break looks it appears to have been a catastrophic break.
I'm sorry I can't help but I'm interested to know more about what caused your Brompton to fail like that.
Is it cro-mo?
How did it fail,were you just riding along and it happened or did you hit a pothole?