Honestly as long as they have a viable Navy, they do not need an Army.
The Royal Navy hasn't been a viable Navy for decades. Even with the HMS QEII, they are a 3d rate seafaring nation. As for the Army, they are already less than 1/2 the size of the U.S. Marine Corps. Like our Marines they are abandoning tanks. NATO and us still have real enemies, but if we don't bail them out, they will have no choice but total surrender.