Vessel looks like a 'River' class offshore patrol ship.
No idea what it is doing there.
The Overseas Patrol Squadron (known as the Fishery Protection Squadron until 2020) is a front-line squadron of the Royal Navy with responsibility for patrolling the UK's Extended Fisheries Zone, both at home and around British Overseas Territories. The squadron, with headquarters at HMNB Portsmouth, is equipped with eight of the River-class patrol vessels.
https://en.wikipedia.org/wiki/Overseas_Patrol_Squadron