Did a trip to Guernsey a couple of months ago. Flight plans are mandatory when crossing an FIR boundary, but 'advisable' for any flight where the track will take you more than 10nm from land.
Sensible to carry a liferaft and wear lifejackets. But otherwise, perfectly possible and allowable for a 'vanilla' PPL such as myself. Having the GPS was handy as a double-check to remain on track, although provided basic VOR tracking skills are used it's no big deal. Failing that you could always follow the Sealink.....
