All the British Upholder class submarines had names beginning with 'U' - HMS Upholder, HMS Unseen, HMS Unicorn and HMS Ursula. HMS Gartside, as a fictional Upholder class submarine, should technically have a name beginning with 'U'.
The HMS Vanguard fires a nuclear torpedo at the anomaly. However, the Royal Navy has not had nuclear torpedoes since the Tigerfish torpedo was retired in 2004.
Officer Shaw and Officer Neal are clearly Petty Officers from the insignia on their uniforms. Therefore their title is 'Petty Officer' or 'PO' - not 'Officer'.
The Commanding Officer of the Gartside is a substantive Captain, his uniform indicates. A Captain would not be in command of an Upholder-class diesel electric submarine in the Royal Navy. That would be a Lieutenant-Commander or, at the most, a Commander.
The Royal Marines are part of the Royal Navy, not part of the Army, as Matt states.