As a result of COVID, workers are entitled to carry forward up to four weeks' leave but only if,
"in any leave year it was not reasonably practicable for a worker to take some or all of the leave to which the worker was entitled"
As your employer asked you to take half your entitlement before the end of June, it appears unlikely you could argue it was "not reasonably practicable" for you to take some of your entitlement by the end of this year and you would lose some of the days not taken.