That's unenforceable - you can self certify for up to 7 days. If they tried taking disciplinary action against you for not producing a doctors note (or fit note as they are now called) you could take them to an employment tribunal - and you'd win.
I'm surprised you Union have challenged such a rule.
Last year I did have a day off before a bank holiday, I didn't realise about the certificate rule and was promptly reminded. I had a follow up appointment with the doctor and requested a note, he said he was quite happy to provide one but it would cost £50. Needless to say the company didn't pay and no more was heard of the matter.
There are NO unions in our place of work, the company refuse to recognise them. We once has a global email go round from a Union, within minutes the email system was down (not before i saw the email). It was down for about 3 days and upon return there was no sign of the email (all other mails were present though).
, i had the flu 2 years ago ...over christmas.