The list seems to be designated for both Customs and Special Branch unless there's an exception noted.
Farnborough has a Special Branch exception noted Shoreham doesn't so I guess Shoreham would count as a designated airfield for both.
Therefore you could turn up at no notice, fill in your GAR in FBO and be on your way.
A lot cheaper than Bournemouth or Southampton too!
It may not be coincidence that all three of these airfields now have scheduled flights to the Channel Islands.