Part of the problem in having better tanks before May 1940 is that they are all going to end up abandoned on the roads to Dunkirk or Cherbourg. Unless there is a similar process of improvement for France. Even with the half decent tanks etc they had, without radios and flexibility they were doomed, doomed I tell you.
However if the RAF were flying in May 1940 what they were flying in May 1941... So increasing numbers of Spitfire Vs and Hurricane IIs. No Defiants, Gladiators, Blenheim Is, Battles. It wouldn't have stopped the loss of France, but it would have made the battle of France far more costly for the Luftwaffe and the BoB much easier. Again however without something similar happening with the AdA in France you still have the same problem, a straight German win on the Continent.
The only realistic POD I can come up with would be a more critical response to the Munich crisis. If there was a real chance of Britain and France going to war with Germany over the Czechs, an earlier possible war (earlier in 1939) might (and I emphasise might) made a difference to the urgency around improving things. So you get a Beaverbrook type in the Cabinet to do to in 1938 what was done post Dunkirk. In September 1938 Britain produced 326 aircraft, inc 18 medium bombers, 51 light bombers and 40 fighters (inc first 2 Spitfires). In September 1939 they produced 781, 74 mediums, 112 lights and 93 fighters. In September 1940 those figures were 1341, 163 mediums, 112 lights and 467 fighters. So have the urgency to increase aircraft production in 1938 towards 1939 rising to 1940 levels. If the emphasis is put onto getting the shadow factories actually producing by mid-1939 you can do it. In 1938 the RAF received a total of 371 fighters (48 Spits). In 1939 they received 1324 (431 Spits). In 1940 it was 4,283 (1236 Spits)! If the RAF knows it is getting up to 250 fighters per month in 1939 instead of 100 they can concentrate on Spitfires and Hurricanes, they don't need the Gladiators and Defiants. Focussing on fewer types means that some production facilities can be rejigged for Spits and Hurricanes, eg Gloster (instead of 480 Gladiators more Hurricanes, even SeaHurricanes!), Westland and Boulton Paul (no Whirlwinds or Defiants).
Having enough Hurricanes and Spitfires in service earlier in 1939, means that improved versions, Hurricane Mk 2 and Spitfire Mk V might be around for May 1940, presupposing Rolls Royce can get the Merlin 45 to replace the Merlin XX ahead of schedule. Unfortunately I can't think of a reasonable way of ditching the Battle as the main light bomber of the day. Wiki says that by July 1938 concerns that it was obsolete had become widespread. So stop producing it then and switch production to Hurricanes, using the same Merlin. Hopefully starting the movement towards a Hurribomber earlier as the Merlin improves. Even having more Blenheim IVs might be slightly better than the Battles. Increased numbers of Whitleys, Wellingtons and Hampdens hopefully means Coastal Command would be better off, as well as bomber command. Switching to tactical rather than strategic bombing for the RAF, ala Luftwaffe is going to be ASB without a fatal car crash involving Portal and his cronies.
For the Royal Navy, if a similar process happens late 1938, more Hunts and Flowers might be around earlier. First orders for Flowers OTL July 1939, make that September 38 and the first 25 are in commission from about January 1940. 10 Hunts ordered OTL March 39 and again 10 more April 39, start being commissioned March 40. Order them September 38 and have them from September 39. 25 extra Flowers and 20 extra Hunts in January 1940 helps early convoys. I would love to see LCTs in time for Dunkirk, but since Dunkirk was the POD that created them, I can't think of a way to get them a year early.
For the army, more and better radios a year earlier would be great. Increasing production of everything, especially SMLE and Brens to equip the first and second line Territorial Divisions for earlier in 39 would be helpful. Won't stop Dunkirk, but would mean more equipped divisions in summer of 1940 reducing fear of Sealion. Valentines in 39 and Crusaders in 40 rather than 40 and 41 will help the Middle East. Even the increased orders of A9s and A10s earlier in 39 (to 250 of each) would have given First Armoured Division and First Army Tank Brigade something more to get on with. They'd still be left in France, but may have given the Division a better start.