Microsoft
used to do an Xbox Live Gold Family Pack, which offered a cheaper way of having up to four Gold accounts.
Microsoft discontinued it before the Xbox One was launched, as on the Xbox One only one Gold account is needed for all other accounts on the console to have the benefits of Gold.
... Unfortunately, the same does not apply to the Xbox 360 - each separate account must be a Gold account to have Gold privileges.
So, you need individual accounts
I would recommend buying an XBL Gold subscription code online, rather than directly through the console (where you'll be paying the full £40 for 12 months).
e.g. you can get a code for
12 + 1 months from Amazon for £29.99.
I renewed last month by buying a
12 + 1 month code from SimplyCDKeys for £28.99.
Amazon itself can obviously be trusted, and I have bought from SimplyCDKeys a couple of times without issue. Another option that may be cheaper is eBay, as there are many sellers on there who buy codes in bulk and then sell them cheaply... but with that you have the usual eBay risks.