My specialist subject... a few ideas...
You should check out the international section of the supermarket or different foreign/international shops.
Milka and Australian flavours in B&M
Montezumas have some quirky flavours
Cadbury mystery bars, or the popping candy jellybean bar
You can get chocolate roulette where one is a chilli chocolate
TKmaxx usually have some less common/international brands and fun shapes and designs
Old fashioned pick and mix - Wilko has a less retro one
Drumsticks chocolate
Terry's chocolate orange popping candy
Tiger stores sometimes have random chocolate treats
Hotel chocolat as mentioned
Blonde chocolate / Caramilk
Lidl - they have a nice one with cornflakes and a white chocolate coconut one (own brand stuff)
Lindt - their stores and large Sainsbury's have large pick and mix with tons of flavours, particularly their standalone stores
Hope that helps!