An oil finish on a pre-war British boxlock is not responsive to "cleaning" other than a wipe down with a damp cloth to remove blood and mud, and maybe an old toothbrush to clean gunk from the checkering. Anything else will just damage the delicate varnish that remains.

Other than the above suggestions, just use it and wipe it down with Clenzoil after each use until you're ready to have it refinished.
JR

Be strong, be of good courage.
God bless America, long live the Republic.