This is likely a fine substitution.
I haven't tried King Goblin myself, but looking around online, I see some good signs:
The description of the beer on drizly (presumably provided by the manufacturer) mentions chocolate malt, and goes on to describe it as:
an indulgently rich, full, smooth beer
The reviews on beeradvocate consistently mention flavor notes such as:
- "Dark fruit" or raisins
- Toffee malt
- Burnt sugar
These are many of the same qualities that you'd be looking for in a stout used for cooking.
Finally, this discussion thread from a homebrewing forum describes efforts to replicate it by hobbyists, and the comments there suggest that the original beer is not terribly bitter (~20 IBUs). This is good news, as particularly bitter beer is the primary thing you'd want to avoid when using it as a dessert ingredient.
It won't be perfect, as an English strong ale probably won't be as sweet or as thick as a proper stout, but I doubt it'll be different enough to cause problems for your pudding.