Except there WAS water between them, which is the exact reason the British Army wasn't that strong in 1939... but their navy was the largest and most powerful in the world: http://www.naval-history.net/WW2CampaignRoyalNavy.htm
not to mention the RAF was again world-class... why would you concentrate spending tons on an army when any invasion would need to attack via the SEA? If there was no water, then obviously everything would have been different and im sure defence spending would have gone more towards a land based defence force so your entire 'what if' scenario is completely null.
Despite RAF, the Wehrmacht was in EUROPE, the British Army was in GREAT BRITAIN AND ITS COLONIES, it wasn't concentrated in England only.