I think that's a fair point. As a pilot in most airlines you accept that you will be working weekends and holidays and earlies and lates.
If I wanted Xmas or NY off that badly I would have chosen a different career. If every weekend off was that important I would not have been a pilot.
I think working into your day off should attract a payment for a full days work. The reason being there are many hours of disruption and delays which don't go into your day off, which don't exceed duty limits, and cause all manner of inconvenience to any evening plans.
Working earlies particularly last summer I would regularly (at least half the time) be getting home 3-4 hours later than I was rostered to be off duty. No kind of disruption compensation for crew. The number of cancelled plans or even just being able to have dinner with the missus or see the kids before they go to bed, all of those plans scuppered with no disruption money in return...
So £500 for going 1 minute into a day off may sound very good on paper, but in reality it's not just 1 minute, but all the hundreds of other minutes that didn't attract any kind of payment. And those kind of minutes you can't refuse to work (assuming you are fit to operate)