How thick is the tube wall of an english axle where the axle brackets are to be welded on. It would be nice to know before I find out myself with too much current.

Paper thin -- it was thin to start with but has been exposed to years of road salt.
The good news is it is the top bar of a "T" weld so heat is drawn from it twice as fast.
Chamfer the edges of the brackets and if you can arange a tiny gap between the bracket and the tube.