Hola!
If I was doing '1938', 'ECWC3', 'VBCW' or whatever we are calling it this week, I would be using British WW1 figures for regulars as there was hardly any difference between the 1938 army and that of WW1, except the vehicles they used. For militia any civilian type figures, even Spanish Civil War.
Berets were not common on the average British man of the time, but working man's caps ('flat caps') and other types were common. The English did not have any hat which looks like the 'Gorillo' at this time. Having said all that, at 15mm you will get away with little discrepancies. Here's the manufacturers I would be looking at;
http://www.peterpig.co.uk/http://thelatequeen.com/http://www.quickreactionforce.co.uk/catalog/index.php?cPath=24If you are interested in the period, a lot of discussion goes on at
http://gwargamesp.18.forumer.com/index.php?act=idx Not many people are playing in 15mm as far as I know, but there is no reason why not. You'll find a lot of information there which will help with forming your army.