From The Observer’s Book of Sea Fishes (revised 1972, copyright Frederick & Warne Co. Ltd.):
… the identification of all rare fish should be checked by an expert, and the staff of the British Museum (Natural History), South Kensington, London, are always willing to help with difficult species, provided the specimens are properly preserved. It is absolutely essential that the fish is preserved in methylated spirit or dilute formalin (both can be obtained from a chemist) for three days and then packed damp, in a polythene bag. Only then may it be posted. Neither the experts nor the Post Office like a mass of rotten and stinking fish. All that is asked for in return for the help in identifying and commenting on your fish is that these few and simple instructions are obeyed.