we never sussed what it was but there have been basking sharks and makos sighted around the west of england. Basking sharks being quite common and a mako being sighted a couple of times.
when i first saw the shark all i actually saw was the typical fin out of the water and thinking it was some person playing a joke i slowly motored over to take a look, as i got closer i realised very quickly it was a damn big shark. the distance from the main fin to the tail was a long way. it submerged when we got closer and i went along side it for a while. I started to call out to other boats and folks swimming that there was a large/huge shark in the water but most of them just looked at me as if i had landed from the moon. a couple of guys were charging around being towed in toys so i told them and they carried on so in my mind i had done what i could and was of the opinion i had told em and if they were eaten then their tough luck!!!

some others got out of the water but you would be amazed how many people just stayed in and shrugged their shoulders
our little boat was 3.4m long and the shark was longer than the boat by some feet
we followed it around and hasselled it into deeper water where we lost sight of it but initially it was cruising in less than 10 foot of water.
it did not have the fin arrangements of a basking shark so perhaps it was one of the makos....who knows.....but for sure i would keep my distance in future especially with my son on board...we are not snacks!!