Paddy is exactly right! The Battle of Britain coin was issued in sets and folders only initially (last year I think it was), and that was before the change of portrait. The circulation pieces issued more recently all have the new portrait. So it's not an error, but it is a valid variety!
Regarding the lines over the face swimming 50p, that too is not strictly an error but a (very rare) first issue before a change of design. The coin once minted proved to be too indistinct, so it was hastily redesigned with the lines removed and then issued in big numbers. Not sure how many first issue rare ones there were released, but certainly doesn't seem to be many. They tend to go for £800 plus.