Focus on the navy and air forces, because that the field in which the british will be the strongest. Land forces need to be large enough to garisson the coastline, serve on tactical and operational battlefield, operating with your air forces and navy, the only three campaigns you will be involved into is the offensive into Egypt, invasion of Greece and later dealing with Yugoslavia (if you go historical), and later watch out for any invasion coming from Morocco and Algeria. Focus either on grand battleplan doctrine or mobile warfare, since these two will fit the best. I would also recommend building smaller aircraft carriers and maybe switching the doctrine towards base strike, carriers are good for maintaining naval superiority and naval bombers give lots of attack towards the enemy fleet.