(UK) A person who is as bent as a nine bob note is dishonest. The reference comes from pre-decimalisation in UK (1971), when a ten shilling (bob) note was valid currency but no such note as nine shillings existed.
The golden rule is the most essential or fundamental rule associated with something.
Originally, it was not a general reference to an all purpose first rule applicable to many groups or protocols, but referred to a verse in the Bible about treating people they way you would want them to treat you, which was considered the First Rule of behavior towards all by all.
If something is worth doing then it is a case of in for a penny, in for a pound, which means that when gambling or taking a chance, you might as well go the whole way and take all the risks, not just some.
(UK) If you look after the pennies, the pounds will look after themselves, meaning that if someone takes care not to waste small amounts of money, they will accumulate capital.
('Look after the pence and the pounds will look after themselves' is an alternative form of this idiom.)