Only basemalt and sugar for milds was more a pre-ww1 thing, during and after ww1 when gravities fell and brewers definitely moved away from Chevallier, Crystal became more and more standard. Not anywhere near the amounts many US-ians seem to believe though.
During the 40's and 50's a small part of roast became common in some brewers milds, Lee's for example hade a period in the 50's where they put ~8% brown malt in their best mild lineups.
But in the late Victorian era (~1880-~1900) you could see some brewers beginning to get more experimental with their milds, such as the one I condition atm, 1885 Kirkstall L, wich contains a notable part brown malt, and occasionally some crystal. Although pale/mild malt and sugar was the norm.
During the 40's and 50's a small part of roast became common in some brewers milds, Lee's for example hade a period in the 50's where they put ~8% brown malt in their best mild lineups.
But in the late Victorian era (~1880-~1900) you could see some brewers beginning to get more experimental with their milds, such as the one I condition atm, 1885 Kirkstall L, wich contains a notable part brown malt, and occasionally some crystal. Although pale/mild malt and sugar was the norm.