An airborne fortress above the approaches to Brest

How would Britain use a mile-wide island with a medieval castle, suspended in the air about seventy miles off Brest in 1919?

Britain is expected to claim it and develop an air and reconnaissance base, adding air defences and possibly heavy guns. Its altitude makes access and resupply central problems.

The premise is clarified as an island suspended roughly 5,000 to 7,000 feet in the air, rather than floating on the sea. A mile-wide top offers enough space for a runway, particularly along the diagonal. From near Brest, aircraft could watch naval movements and give the position value in a later world war. Air defences would be necessary, and surplus 15-inch guns are considered for fixed emplacements. The island's suspension is accepted as a fantastical premise for the basing problem.