DON'T USE CELLULOSE THINNERS
Thought I had better get that in first, as it will dissolve the resin
As it happens, I have a Bramble Line loco which is almost identical sat on the shelf. Not mine, but like yours, is awaiting a repaint after it was hand painted with Humbrol enamels. Thought I would do a test after Gerry spoke to me yesterday.
I put the roof in some Acetone for a couple of hours and this was enough to soften the paint to the point where it could be rubbed off with a tissue. For removing from all the nooks and crannies, an old toothbrush will probably do the job. There is a drawback though, if the kit was assembled with Superglue, the acetone will also attack this and you could end up with the kit in it's original pieces.
Hope this helps.