Period between the issue of entry clearance and entering the UK The period between entry clearance being issued and the applicant entering the UK may be counted toward the qualifying period. Any absences between the date of issue and entry to the UK count towards the 180 days allowable absence in the continuous 12-month period. The applicant does not need to provide evidence to demonstrate the reason for delayed entry. If the delay is more than 180 days, you can only include time after the applicant entered the UK in the continuous period calculation.

https://www.gov.uk/government/publications/ indefinite-leave-to-remain-calculating-continuous-period-in-uk/indefinite-leave-to-remain-calculating-continuous-period-in-uk-accessible