Some old cruisers could be grandfathered into the treaty fleet

Did Britain consider keeping any armored or protected cruisers after the First World War?

Yes; some could be retained and grandfathered into the treaty system.

Postwar scrapping was not an automatic decision for every armored and protected cruiser. Some ships were considered for continued service. Treaty arrangements could grandfather existing hulls into the new system even when comparable new construction would be restricted. That option gave old ships residual value while navies adjusted to the postwar limits.