If you look more closely, you may have noticed that some camellias hold on to their flowers until they turn brown while others shed their coloured petals everywhere once the blooms mature.
There are lots of different camellias but the ones you see in London gardens are nearly always varieties of the Japanese camellia (Camellia japonica) or one of the Camellia x williamsii hybrids. The former hold on to their dead blooms, the later drop their petals.
Which to choose? I would always go for the one that looks without the need for deadheading. But, if you have a bad back or don’t like sweeping, the Japanese types may be your taste.