To add some perspective to how fatigue is managed at 2 other UK airlines I've worked for.
Call crewing.
"Im fatigued for my duty"
"Ok I'll take you off"
"Thanks bye"
And you'd never hear about it again from anyone. No follow up phone call or email. All you would have to do is fill in a fatigue questionnaire as to what caused your fatigue. And anything from jet lag, to a night flight to a screaming baby at home are perfectly acceptable reasons. In fact, you're completely untouchable when you go fatigued. More so than if you call sick.