The steep banks around this pool, and constant depths, make this a distinctive feature, and it is unlikely to be wholly natural. The only natural features in this country that are steep banked holes of the right size would be kettleholes, which were left when mud-covered ice from the last ice age melted ~10000 years ago, and sink holes, which are formed by water running into limestone. The former rapidly loose their steep banks (certainly by the climatic optimum ~6000 years ago) and fill with sediment. While they can punch through upper sediments to aquifers and fill with water to produce lakes with no obvious outlet, they are unlikely to be consistently knee-deep. Sink holes are usually deep, and are unlikely to contain standing water.