Davy wrote:

San Diego Joe wrote in
:

Egeria densa

unfortunately in the UK that is an aquarium plant; perhaps
its too cold here!

Then try its cousin "Elodea Canadensis", which survives in heavily
ice-covered ponds. E. Densa is native to Brazil, so probably not best
suited for overwintering in the UK, but should do fine as an annual.
--
derek