Used Bimbling since the 70's at least.
Possible it may have military usage as well.
I understood it to be a slang military term, meaning to walk casually or slowly, perhaps without any clearly defined purpose or destination, and whilst off duty.
Quite different from walking smartly or marching from A to B whilst on duty.
An NCO whom I know was on duty in London for the Olympics, and mildly reprimanded men under his command for "bimbling around, like civilians, rather than walking or marching more purposefully"
To bimble implies a bit more purpose and planning than to bumble, but not much.
Example "I went for a bimble around town, undecided as to which pub to visit, or perhaps go the curry house"
In contrast to "I walked to the station, making only a very brief stop to purchase a newspaper, knowing that I had to catch the 08-32 train"