You cannot simply hand your practice over to a new partner, so I hope that you have checked this all through with your PCT. If you have not done so, the practice may be put out to tender by the PCT when you retire, whether or not it is 24-hour retirement.
To be absolutely certain of the succession of the practice to the new partner, you would need to give the PCT 28 days notice that you are taking on a new partner. Then, on the day that you take on the partner, you can give 28 days notice of both your retirement from the practice and that the practice will be going back to being single-handed.
If you take 24-hour retirement, you only need a break of 24 hours, but you must not work for more than 16 hours per week in the first month. Thereafter, you can work as many hours as you wish.
There would be advantages in coming back as a partner. For example, the practice could not dispense with your services whenever it wanted.