The term 'unmeasured working time' is a bit vague, but is intended to apply to senior managers and executives who genuinely have control over their own working time. These workers are entitled to paid holidays but don't receive other working time rights. Workers who are free to set their own hours are deemed to be in a strong enough position to look after themselves and are thus exempt from the working time limits and rest break provisions.
The problem is that many managers in workplaces with a long-hours culture still behave as though this exemption also applies to all white-collar workers, for the so-called 'voluntary' work they 'choose' to put in over and above the hours specified in their written contract document (and for which they aren't paid). It does not. You should take further advice if you are concerned about unpaid overtime.
Many unions campaign on the issue of unpaid overtime. Every year in February, the TUC marks Work Your Proper Hours Day, which is the day on which the average person who does unpaid overtime finishes the unpaid days they do every year and starts earning for themselves. WorkSMART also has an unpaid overtime calculator, which you can use to work out how many hours of free work you are giving to your employer.
If unpaid overtime is an issue where you work, and you are not yet a member of a union, browse our Union Finder tool to find out more, and read about ways of organising with your colleagues to enable the union to seek recognition where you work. A recognised union can negotiate improvements in your pay and hours, and the more people that join the union, the stronger its voice will be.