For sure you could add the Martlet in a later set, but the spring '40 events were all Seahurricane's, Gladiators (which we dont have obviously and dont have a suitable substitute) Hurricanes I's and Skuas. I dont think the Martlet I's had been aquired by the FAA yet as they were diverted from the French after they surrendered, but you could add them for sure if folks felt they were really needed.
The more I think about it, the more interesting it sounds. The planes are all fairly evenly matched, with the edge probably going to the LW, but its the opposite of the BoB, the RAF will be the ones with the long flight to the battle not the LW.