The coastal bits of Cornwall (particularly North Coast) aren’t that well served by bridleways but given I live on NE Dartmoor and am only an hours drive from the North Cornwall coast, and Cardinham is just down the road there is plenty of riding within striking distance if you don’t mind a bit of a drive… In my experience cheeky riding is a bit more tolerated generally in Kernow, especially if it’s early morning / evening 😉
Some good recommendations above. Polzeath is a good holiday base and has a great beach which is a safe choice for surfing – lovely walks on the coast path out to Pentire Point and beyond too, but it does get rammed during the holidays.
Our favourite beaches on the North Coast are Trebarwith Strand, Crackington Haven and Widemouth.
Further West, St Ives is lovely, with loads going on and great beaches. Sennen Cove is stunning – somewhere like St Just is central for that area and may be a good base if you want to avoid the crowds and also get some riding in – there are a few routes (some cheeky) around the Penwith peninsular.
St Agnes area also recommended – some great beaches nearby and handy for the Mineral Tramway which runs from coast to Coast from Portreath to Devoran and apparently has some off-piste stuff to explore in the old workings.
Some lovely spots on the Lizard (Kynance is gorgeous), and bits of it can get EPIC surf, but conditions can be a little inconsistent at the intermediate / beginner friendly beaches.
Have a look here for cottages: http://www.helpfulholidays.com/