The odd thing is that they used to be much more adventurous in the past. Samuel Pepys was exceedingly fond of oysters, which means there was a ready supply in Charles I's time, and nowadays they are mostly exported to the EU, as are scallops and lobster. The standard of British food has improved markedly since the 90s, with the rediscovery of classic English fare by a new generation of chefs, if they can rediscover samphires, you'd think they could also rediscover shellfish.