Gulls are known to crack open mussels and similar prey by dropping them onto rock. I have never heard of a gull doing anything similarly constructive (or perhaps destructive) on sand.

I can only speculate that you were watching a young or inexperienced gull that had not yet learnt to select a hard surface for its nut-cracking activities.

I've only seen crows doing this. In Ireland it's the hooded crows that do it, and they normally have the sense to drop it (usually a mussel) onto a rock. In South Wales you'd be looking for the all-black carrion crow. Perhaps your gull is showing learned behaviour, but, as Jon says, without the nous to factor in a rock.

It might be interesting to email the BTO and mention this because they are always interested in unusual bird behaviour. If you know the species, all the better.