There is no requirement for this but anybody crossing the Channel should carry a
raft.
A PLB won't do you much good unless, possibly, you have an immersion suit and are happy to be bobbing up and down, for possibly many hours, with your head sticking out of the water, and your arm holding the PLB.
You can now get life rafts down to 7-10kg - Survival Products, from e.g.
Harry M. I always fly with one of these.