2nozee
Our company (perhaps the same one, MON) has the same rule and I'm pretty sure it doesn't apply to flightdeck; or at least it hasn't been pointed out to me.
This rule was first introduced to ensure that everyone turned up 'smartly' even though most of our positioning is done far from the eyes of possible passengers. These days ties are being slowly removed by upper management and indeed some UK cabin crew uniforms. Being 'smartly' dressed does not mean wearing a tie these days.
No doubt our company will change their mind but if past history is a guide it'll take some time. All they need to say is don't turn up wearing clothes like Beckham!