If you take Business and accountancy as your example, you can see here
how the years are laid out.
You can see in first year you have to take Business Studies 1, Accountancy 1 and then your options are either Economics 1 or Economic Principles (so no choice of Spanish, but you are taking 3 separate subjects).
In your second year you have to take Accountancy 2 and Business Research methods. You then have an option of picking 2 or 3 Business courses from a list. If you only take 2 business courses you can pick any 1 20 credit course from the Schools listed here
as schedules A-J or N-P, which covers any humanities school or GeoSciences/Informatics/Mathematics. (At this point you are picking from 2 or 3 subjects).
Your 3rd year has 100 credits worth of compulsory modules which all come from the Business or Accounting departments. You then have the option to pick either a further module from Business (see here
, the subjects labelled level 10, modules available with change year to year) or the Ec&Social History module "British Business History Since 1930". Thus you could take modules solely from Business and Accountancy or you could take a module in a different subject but related subject material from a different department.
And your fourth year only has the compulsory dissertation (which you will choose the subject matter for) and then 80 credits of modules from the same selection as above.
Overall you are studying Business and Accountancy subject matter but you occasionally are allowed to venture into other subjects and departments (and with other degrees where you aren't aiming for an accreditation at the end there is more leeway in outside courses and subjects taught across different departments).