Hi,
my suggestion is to identify the research groups you are interested in and to check quite often their websites.
Secondly, what aspects of ecotox would you like to study? At plymouth they are very good in invertebrate ecotox, at exeter fish and genomics, Brunel Univ fish and reproduction, Birmingham metabolomics.
may be it would be better if you consider aquatic ecotox rather than marine ecotox, just to have more choice. Infact in the Uk the majority of ecotox research is related to freshwater fish.