Can a young worker decide to work more than the limit imposed by the Working Time Regulations 1998?

No, a young worker must not be permitted to opt out of the maximum weekly and daily hours limits imposed by the Working Time Regulations 1998 (Si 1998/1833). Young workers under the age of 18 may not be employed for more than 40 hours in any week (including overtime hours) or for more than eight hours on any day.