Number one must be the Dixon range in 15 mm since they have the 'best' range for both sides. You will then be visiting the Essex and QRF Mogul (and may be some other) ranges to get some additional variety. Irregular have some useful figures but they are bigger. Not familiar with the Lancashire range but I think they might be a little short and 'squatty' but I could be wrong.
No reason why "I" would say this, but you can't go wrong with 15 mm. In 28 mm you'll need a gymnasium to do what we can do in the smaller scale. No offense to the Steroidal Scale enthusiasts - or even their manufacturers!
But, again, start with Dixon's range and see if you can get all you want there. Depends on how big and how "wide" you go with your collection. Just some of us - like me - go a little crazy in accumulating figures.
Whichever way you go, have fun. And get a copy of Wargames Illustrated issue 48 for the list of uniforms - in text but a good resource at a fair price.
Some will charge you a lot for less information, sad reality of our hobby. I don't mind people making a fair profit but it irks me that some people will take a small amount of information and then subdivide into even smaller parts for no other purpose than to gouge the hobbyist. (Now climbing down off my soap box.
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