Absolutely not. If I told you to go to the apartment on the eighth floor, why would you press seven? This has no relation to how a building's floors are numbered. I agree that what would be confusing would be on the way down and how the ground floor is labeled. If that is what you meant.
It depends on whether I want to describe how high something is or identify the floor for giving directions and on the audience.
If I'm taking to another Brit about how high up an office of flat is I'll use the UK system, but if I'm giving directions - especially if there is a lift involved - I'll use whatever the local system is.