As NJ states, there's no legal meaning to 'next of kin' (and definitely no meaning to '
named next of kin). The ONLY way to name a specific beneficiary (or a specific group of beneficiaries) is by writing a will.
If your father dies without leaving a will then, by law, his estate MUST be distributed in accordance with the information about 'intestacy' in the links above. If his estate is small then the relevant bank(s) and/or building societies MIGHT not require anyone to apply for 'letters of administration' (which is a legal grant, equivalent to 'probate' where a will has been left, giving the holder the right to access and distribute the estate of the deceased). They all have different rules though; see here:
https://www.co-oplegalservices.co.uk/media-centre/articles-may-aug-2018/bank-limits-for-probate/
If the estate is larger than the limits shown above, then a relevant person will need to acquire 'letters of administration' before the funds held by your father's estate can be accessed and distributed. (The application needs to be made to the local Probate Registry).
Money held in a joint account with another person automatically passes to the survivor following the death of one of the account holders. However adding a second person to an account does, of course, open up the risk of them clearing out that account while the original account holder is still alive!