No a FP is not required as the SI are within the UK. You can file if you wish, but the sea crossing is very short and you will be in radio comm. the whole way.
The usual comment about the SIs is the runway is a little different siting on the top of a dome shaped hill as it does. This means you cant see half the runway and so it appears a great deal shorter than it actually is. The perspective is also different. The cliffs can also make for some sporting sheer and turbulence if some conditions.
It is a great flight and well worth the trip.
You can give Culdrose a call if you wish but they arent usually operational at week ends - just check the NOTAMS.