New One Pound Error Coin / Misprint

You have a partial collar error, nice find The collar, aka the "third die", forms the edge of a coin while it is struck between the obverse and reverse dies. If the collar is not in place when a coin is struck, the planchet expands to a larger than normal diameter and also lacks the security edge/reeding that is applied by the collar. On your coin, the collar was partially engaged so half of the edge looks normal while the other half was not contained by the collar and expanded outward. I am not familiar with the values of UK errors but it is definitely a keeper.

First thing that is a definite valuable mint error. I would clean it by giving it an acetone bath, rinse off with distilled water, pat dry. Put it in some form of holder, 2x2 or flip, and Hold by the edges only!