These days with the advances in medical science whilst death is still a risk, one also has to consider incapacity whether mental or physical as a potential outcome so I'd advise registering for Lasting Power of Attorney for both property and financial matters - two separate but similar documents -
https://www.gov.uk/power-of-attorney
It's a bit of a blunderbuss to execute if the event comes to pass but simple to put in place and ensures your chosen person can carry out legal and financial transactions on your behalf and hopefully in accordance with your wishes too.
Having those documents in place means that institutions are legally obliged to take instructions whether financial or property related by your chosen attorney who is acting on your behalf, and thus obviating the need for duplicate access/sharing of account details with a partner etc.
Whilst you might think otherwise it is the correct and legal way under UK Law to allow financial transactions upon a deceased or incapacitated persons accounts.