Coming to this late, but this is typical behaviour for the Long-tailed Tit.
These birds are almost always seen in groups and stay very close together. They roost and perch on wires or thin branches, staying very close together.
You talk of a black cap. Long-tailed tits have black markings on their heads but with a white centre.
In the sea
Most marine wildlife here is harmless, although Orcas do visit British waters. Although they are not generally interested in humans, it wouldn't be good to be mistaken for a seal.
Jellyfish appear in large numbers at certain times of year. Entanglement can cause severe shock and require hospital treatment. It can also affect ability to swim, ...