Being named in the will does not actually make you an executor; nobody can execute any will until probate is granted (both legally, since somebody else may wish to put forward another will, and practically because no bank or asset-holder will give you the time of day without a grant of probate).
When the single executor applies for probate the relevant website says
If more than one person is named as an executor, you must all agree
who makes the application for probate.
If only one executor applies they’ll need to prove they tried to
contact all executors when they apply.
so the simplest thing would be for the 'passive' executor to write a note saying he is happy for the estate to be administered by just one person (if that is his wish) and for the active executor to attach the note to the application form.
Of course, he may actually wish to be a formal executor but leave the other person to do the work. The Ministry of Justice has no objection to this so long as the two can agree; in that case both would sign the application and be jointly responsible for the estate.