Employers have different ways of handling things when people work extra hours. The standard is to give overtime, but some employers may offer time off in lieu instead.
You may be happy to take it, especially if doing extra work is a rare occasion for you and you are just lending a hand. Yet, if you do not understand the rules, you might be giving away your time too cheaply.
The overtime rules
Employers must give time off in lieu at the same rate as overtime.
Time off in lieu may suggest a like-for-like swap. For instance, you work an extra day this week so your boss tells you to take an extra day off next week to compensate. Yet the same ratios that apply to overtime pay must be applied to time off in lieu.
The standard ratio for overtime is one and a half. Sometimes the ratio rises to two times the extra amount worked. Those same ratios will apply for time off in lieu.
One and a half times: This applies to the hours you work over eight in a day and those on a seventh consecutive day of work.
Two times: This applies to the hours you work beyond twelve in one day.
If your employer has not applied the appropriate ratio to time off in lieu they told you to take, it might be an oversight on their part. Or, it could be they are deliberately trying to underpay you. Approaching them politely to point out the issue may be enough to resolve it. If not, you may want legal guidance to explore further action.