Elgar, Britten, Vaughan Williams, Purcell, Holst>Russian
I found this trickier after the first four:
Tchaicovsky, Stravinsky, Rachmaninoff, Shostakovich... Mussorgsky>German
This is easily the toughest, with Germany only being 200 years old.
Wagner, Schumann, Mendelssohn, Brahms, Strauss.
What was supposed to be obvious? The British one was the easiest of the three.
Furthermore, you started with "the arts" and then shifted the goalposts to "classical composers." If we go with poets or painters the margin by which Britain is superior to Germany and Russia is even bigger.