The Isle of Sheppey lies in Kent, on the edge of the Thames Estuary. And while it may not be viewed as a classic sea kayak circumnavigation, bordered with industry and caravan parks as it is, it still does tick various boxes for a performance paddling challenge. At 43 km it is long enough to challenge but short enough that it doesn't take over your life, and of course good planning is required to match the tides to a successful lap. We reckon it's worth adding to your list.
Image: Estelle Kemp