If you want Amph ships, etc, you need a good sized marine establishment!The problems the British Army had with its equipment was that development was halted for 6 months or more in the post Dunkirk panic. Once the French collapse the BA can't stay in France. To prevent the panic you need to rescue the equipment with the troops so somehow the British have to have a supply of Amphibious Warfare ships and landing craft to pull them off the beaches. So the post WWI planning will have to accept the lessons from the Dardanelles and recommend the purchase of the appropriate shipping.
You need to RM out of capital ship turrets and back on their feet. The merge of RM Artillery and RMLI in ‘22, the RM lost direction. With a cap of 9,500, it could not do much, either.
In ‘35, you need the RM Div, as a full all-arms unit. Gunner stream with guns, and the LI stream in rifle brigades. Each RN base have a Defence brigade, with coastal guns, infantry and RMR to rapidly expand these units.
Combined with 35 NTA, (6 mth conscription then RMR), each formation, can be at 50% manning, easing pressure on budgets.
Considering much of the Landing Craft and above of GB were built in the US, RN can only expect modest Amph fleet units and minor craft.
Older passages ships converted to LSI, and USN style “assault destroyers” would be more realistic.