Is it a custom in the UK for all ships in a particular class to have names
that start with the same letter as the class of ship? Seems to be.



Well yes and no, in the past some Classes have all started with the same
letter, especially submarines, but it is more common to have a 'theme' for
the names: eg County Class - names of counties, Leander Class - Classical
Greek heroes Battle Class - famous battles etc.

The 'Old' Daring class ships did all start with the same letter.

Regards
Jeff