Testing for the presence of Hexamethylenetetramine / Formaldehyde
I don't believe the premise that E239 (the E number designated to hexamethylene tetramine) is used to preserve fish within the EU is correct - in January, 2014, the European Food Safety Authority published an article in their journal about this preservative where it states quite clearly that the only permitted use for this additive is in Provolone cheese. There are concerns that over-consumers of this particular cheese may accrete more levels of E239 within the body than are healthy.

E239 does appear to be used in the UK for preserving fish, or at least, it is listed as one of its intended uses. Whether its actually used or not I haven't been able to determine, but given we're in the EU, technically, it shouldn't be. If I source any other research regarding this, I'll add it later, but currently, it seems it depends where you're buying your fish from in the first place, that is, the country of origin.