Hello. Welcome. How wonderful to have a chance to explore the Welsh coast.
Hard to say what that is. Just how small is that?
In America I would be wondering if it were a Christensen Agate from Ohio or a swirl made by one of the many West Virginia company. It is true they exported to the UK but it's harder to feel confident about guesses for marbles found in other countries.
Anyway, getting a good feel for the measurement would be a good start.