Probably the same euphemism that has chicken in the fried chicken / beef in the burger?
Don't bother with it in London, elsewhere go where the locals are. A busy fish and chip shop in a small town is probably best, rather than a pub or a takeaway that also sells pizza/chicken/kebab.
I don't live in the UK any more, but last time I visited the fish and chip shop in Llanfairpwllgwyngyll. Highly recommended :)
