Upvote:2
Per TIMATIC, the database used by airlines:
Passport exemptions: Passengers with a Seaman Book issued by Azerbaijan, Brazil, France, India, Luxembourg, Moldova (Rep.), Russian Fed., Spain, Tanzania and Tunisia (which have ratified ILO Convention 185) or issued by countries that have ratified ILO Convention 108. The Seaman Book must contain the name and title of the issuing authority, date and place of issue and a statement that the document is issued for the purpose of the relevant Convention. The seaman must travel on duty.
The UK is a signatory to ILO Convention 108.
As such, you don't need a passport or visa to enter the UK