ID    month    country   count    style
1     2012-02  UK        3        high
1     2012-02  US        10       high
1     2012-02  FR        5        high
etc

aggregated_data = setDT(subset)[, .(Country = list(Country), ID = min(ID),
count = sum(count), by = list(Model, Month)][]

ID    month    country     count
1     2012-02  UK, US, FR   18
etc