Background
By 1918 Britain had learned some bitter lessons about modern littoral or amphibious warfare. As early as 1917 the Brits were building first generation assault landing craft, floating docks, & other accouterments for beach assaults and littoral warfare. Why these lessons were not better preserved and built on is a interesting question I'll leave to experts.
The PoD
Post 1918 British begins, like the USN to preserve and build a specialized expeditionary/littoral warfare force much like the USN did in the 1920s. This need not be a large force, a couple of brigade like the US Navy formed of its Marines. What are the potential knock on effects of this from the 1920s.