Given there is far more coastline than available landing strips how do the Navy stop small boats offloading their 'cargo' at any point where water meets land? Is the entire coastline covered by surface (sea) radar?

Looks like only 3 boats for over 7000 miles of coast line. I don't think this enough - unless there are other means. Are there any?

Royal Navy has 'worryingly low' number of border patrol boats guarding UK coastline | The Independent

UK Border Force don't do themselves any favours when it comes to 'regulating' who lands via a 'small' airfield. A form (General Aviation Report or GAR) is sent to them in advance and if there's no-one on it who is 'suspicious', they either don't attend or (if the passenger is non EU) fax a visa to the airfield for someone to hand to the passenger. But what if the airfield has no fax machine or a 'responsible' person to had over the visa? And what if there is a 'late' additional passenger not included on the original GAR?
Many licensed airfields are totally unmanned after closing time in summer well before it gets dark so it would be easy to land, disgorge a few people, and depart again.

Nobody here heard of Project Pegasus ?

I know in Cornwall there are volunteer spotters who are out and about with field glasses and phones a lot of the time

3Wheels, in a word No, at least not until you mentioned it. Unlike the RN shipping one, whose name escapes me, this appears to be an internal project and not yet an operation.

There used to be a joke that at the end of every military runway was a farm or cottage for the duty spy. Maybe modern strip owners are in the project but of course some might be complicit in the smuggling.

When we leave the EU maybe cig smugglers will return too.

Hornbeam - Somewhere I still have one of the cards that were distributed with the Cyrillic alphabet and the text "What, Where, Whither, When" on one side and a free phone number one the other.

YS

YS, that's the one. I filed one from ASI, visual on a Bear.

What about this charity?

UK Civil Air Patrol | Providing air observation to save lives across the United Kingdon

cheers