All joking aside, I would get some WD40 or swarfegajizer into the threaded end then shock the thread loose starting with a good solid 'twatting' with a mallet end on to the tube then a gentle tapping onto a bar that is a tight fit into the end slot, back and forth.
I have never had a webley pistol that would not come apart and I have stripped a lot of them...
"But we have our own dream and our own task. We are with Europe, but not of it. We are linked, but not comprised. We are interested and associated, but not absorbed."
Winston Churchill 1930