It would be wouldn't it? The June (Winter) Solstice is tomorrow. The fact that the shortest day of the year is right upon us should be something of a sign that it's not really swimming weather.
The fact that everyone is walking around in overcoats and scarves is probably also a fair indication that stripping off to a bikini and taking a dip is somewhat unusual.
But the lifeguards were on duty at South Bank and if you are brave why not?
Apart from the obvious.