I was reading all about the British Record (rod caught) Fish Committee at
http://www.nfsa.org.uk/brfc/brfc_procedures.htm and came across this -

The following are rare or threatened species in Great Britain and are
protected under the provisions of the Wildlife and Countryside Act (1981)
and later Orders. This protection results in it being an offence to capture
any of these fishes intentionally. Anglers who believe that they may have
captured a specimen of any of these species are advised to unhook and
release the fish unharmed as soon as possible after capture.
Allis shad Alosa alosa
Burbot Lota lota
Schelly, powan, or gwyniad Coregonus lavaretus
Sturgeon Acipenser sturio
Vendace Coregonus albula

Thought I'd mention it as, if I recall, someone recently said he was going
Sturgeon fishing.