Extract Sixty-Four Coincidentally or not, there is another and highly Megalithic Rat Island:

It forms the northern extension of Hugh Town on St Mary’s in the Scilly Islands. Here there is the triple-lock of Rat Island being a near (previously?) tidal island, it guarding the larger near (previously?) tidal island of Hugh Town and the whole forming the main harbour of the Scilly Islands, the westernmost point of Britain.
Rat Island in the Scillies has lost its status as an island but at least retains its name. The reason given for Rat Island changing its name to Burrow Island is that it is now part of the Borough of Portsmouth and not Gosport to which it is closer. This is an explanation that gets the horse and the cart somewhat confused since it rather more likely that Portsmouth is a borough because of Burrow Island!
The reason this island and St Michael’s Island along the coast have their alternate names is because they are ‘burgh’ islands, a burgh being a very ancient name for a trading post. Burghs eventually became a Europe-wide phenomenon (and an entire branch of the place-name industry in its own right) but for our purposes it is enough to observe that whenever a burgh derivation comes up in an offshore island context, the hand of Megalithia is sure to be found.
Here for instance is Burry Holmes in the Gower Peninsula, Wales

which has all the characteristics of being not merely a Megalithic causewayed tidal island but to be wholly artificial. The flat top and the trailing islands would seem to indicate that it was either built from the ground up or has been strenuously resculpted. It has the usual mixture of pre-historic settlement evidence and a medieval monastery but not enough local resources have been deployed to say more.