Possible Dolphin? Not sure.
No interactions present.
If it was a dolphin or whale, the +- rigid fin would be rapidly moving, and the animal would likely break the surface frequently. Basking Sharks move much more slowly, and rarely show the back, though usually the snout and tail-tip will be visible at times. The BS fin is floppy and the tail sweeps side-to-side (in dolphins it goes up-and-down). That should help you decide between shark and dolphin/whale, but not the species of dolphin which needs more than just the fin. My guess would be Basking Shark.
recording wildlife with The Recorder's Year on www.hbrg.org.uk/TRY.html.
Lat/Lng: 50.22, -5.4967
OS grid ref: SW506414