Could there not be a reason that Apple made this choice involving their own legal risk? Sometimes what a law actually requires is up to what happens in court in the future.
Part 5 is too broadly written: https://www.legislation.gov.uk/ukpga/2023/50/part/5
'internet services' is extremely broad and could include apple's own appstore, icloud services, maybe even their browser could be considered software acting on behalf of a provider.
Now of course they could be stretching, but OFCOM has their own overview that digs into just how broad they consider the legislation: https://www.ofcom.org.uk/siteassets/resources/documents/onli...
With all this being said, I do think Apple probably could have fought it and even if they had to leave the UK market, they'd still be fine. They rely on China and South Korea to manufacture their devices so they would not be fine without these markets.
Apple doesn't need to fight anything.