The Bank of England guarantee only applies to Northern Rock, not other financial institutions. Plus new accounts will not be guaranteed; only those customers who had an active account before midnight on 19th September. Furthermore, guarantee is not indefinite. When the financial markets become “more stable”, the guarantee will disappear.
With this in mind, if I had family members/friends who had money and other investments in Northern Rock, I would advise them to move funds elsewhere. Without a takeover by another bank, they are like a wounded animal that needs to be put out of its misery.