You have the Concise source book here:
link
The boards (already linked) are a very good rescource.
Remember it is a setting, not a rule set. Find out what players in your area, what scale, what rules. Then start building accordingly.
I recommend your first force be a militia/LDV unit, they will be an all sides, you could use multiple flag carriers to define which side they are on. Then start to build something more on theme.
Tips
It's 1937/38, the Bren Gun and WW2 style battle dress have only just started to arrive with troops. So even regulars will often be in WW1 style uniforms with Lewis guns in support. I said often there, if you have an early WW2 British force, by all means use them.
Anglicans in the south/Kent tend to have some french support and received some french equipment. If you do not wish to go full French in your look (Because, well, standards!), some ranges of figures for WW1 Americans have some figures using French weapons which would suit you (Chauchats and hotchkiss MGs) and still fit in due to wearing the "British" Helmet.
Air forces? I'm using the dramatic conceit of having loyal RAF in WW2 early WW2 schemes, and those that are disloyal to the crown in the interwar Silver dope. Factions of course will have their own quirks with insignia and colours.
bases up the start of his 1:72 scale WWII Russians.
