@Georgie870 I don't work full time so I can't tell you how to do it I am afraid. I WFH part time and struggle enough (no ex to help).
Could you go for a brisk walk in your lunch break to get exercise? Walk home or to work rather than get the bus if that's possible?
Get kids to help you tidy up the entire house two evenings a week where you all pitch in, with a treat after it's done (pizza or something)?
Meal prep at weekends as far as you can (chop veggies, cut carrot sticks and put them in water in the fridge - they last days this way), portion out dinner ready to reheat in microwave or oven on different days. I usually do some meal prep on a Sunday.
One load of washing in the morning or evening each day
Get a smart watch as counting steps really helps me stay on top of exercise a bit more. 2 minutes only of weights or squats or something each day. Keep goals small - it's better to do 2 mins than nothing.
Life admin - spend 10 minutes a day during your break at work dealing with emails etc.
School uniforms and packed lunches - I tend to prepare these in batches - two to three days in advance for packed lunches, uniforms get ready on sunday on hangers with everything you need on each hanger for one day of the week.
Shopping - weekly online delivery with everything you need.
Decluttering - I always keep this in my schedule, as staying on top of everything is so much harder in a cluttered house. I try to clean one small area (a shelf) and declutter it every other day, so you are constantly removing things from the house. Takes 5 mins.
Realistically it's not always going to be possible to do everything though, especially as a single parent. Best of luck!