All British gun and howitzer ammunition be it QF or BL has a certain amount of sheet lead foil or tinfoil included with the propellant, provided it is necessary.
Each projectile with a copper driving band, when fired, leaves a copper residue in the barrel. The foil in the next round fired forms an amalgam with the copper, which becomes brittle and is pushed out by the next round fired, which in turn, leaves its own residue, ad infinitum.
Projectiles which do not have a copper driving band e.g. APDS, APFSDS, Blank cartridges, do not have the foil added.
This has been so from way back, certainly before WWII and may be much earlier.
They are not common, I have only ever seen one box of 40mm, so I had one. Possibly I just was not called upon to inspect any.
Larger calibres have tools for removing stuck fast projectiles but this may not necessarily be so for smaller calibres e.g. 20mm.
Having said this, if the chips were down operationally and there was no way of removing said projectile from the bore by the slow method, you would use whatever was available to remove it.
When I started in the business 6pr, 17pr and 20pr were in vogue, so I have included some corroborative evidence from the period.
The foil is sometimes placed as sheets on top of the propellant, sometimes crumpled up, sometimes in a cup, other times inside the cartridge and other wrapped around the charge. Where it goes is generally dictated by the type of propellant, e.g. stick or granular.
In the case of these 17pr cartridges the clearing charge sheets are double the size of the other cartridge.
All the copper is not necessarily removed by the following shell, particularly under operational prolonged firing, so a clearing cartridge is used.
Another use in earlier times may have been to remove as much copper as possible before making gutta percha bore impressions in the artillery workshop.