A foundation diploma is equivalent to A’levels (academic level 3). A foundation degree is 2/3 of a degree (equivalent to an HND - academic level 5), I googled…
If you’re being pointed toward Cambridge/Oxford then a foundation degree is not you, I would have thought? (A degree is awarded at academic level 6).
If you want to do a degree in the arts, but you are not yet sure in what field/discipline - or you just want to build a portfolio with a diversity of approaches and skills - then take an Arts Foundation at college before applying to University.
Universities now often have a Foundation Year or Year Zero, which is similar to the foundation diploma and - potentially - a ‘feeder’ course to a degree, but you will become liable for degree-level fees (I.e. £9,250) for that year.