Assuming you're still resident for 21/22 and you don't claim any split year treatment in the years of departure or return, you would need 6 full tax years as non resident, i.e. 22/23, 23/24, 24/25, 25/26, 26/27, 27/28. You would then be safe to go back on 6 April 2028.
Here's some guidance that...