As mentioned before, it can take a while for the rectum to reduce after impaction, which affects the sensation to "go".
One Movicol a day is a maintenance dose, on occasion if a child is impacted they should be on four sachets a day.
A lot of parents can't make the link between soiling and constipation as they think the two co tacit each other, but the link I gave previously gives a really good diagram of how this happens.
I would contact your health visitor/school nurse as you will get more support than a GP has capacity to provide.
Golden rules
Comfortable position on loo. Preferably with a footstool, so legs are not dangling.
Regular toileting 20-30 minutes after meals for at least ten minutes, after breakfast is best.
Reading stories/blowing bubbles, special books to look at only when on the loo to encourage sitting on the loo.
Reward charts based on positive things, ie going to the loo after breakfast, taking Movicol, drinking more water/weak squash (about 1000-1500ml a day), doing a poo on the toilet, but never punishment for soiling.
Sometimes gentle massage tothe lower left abdomens can encourage pooing.
Healthy diet, with fruit and veg. And whole grain foods.
Keep a poo diary with time, size and type of poo to see if there is a pattern/specific times of day it happens.
If there is also wetting, the constipation needs to be sorted out first. Then daytime wetting. Then night time wetting,
The Movicol may cause accidents at first, as Movicol takes water to the bowel, lubricating the faeces there. If your child is used to holding poo, they may not be able to do this once on Movicol and the urge to poo and can get taken by surprise.
Unfortunately it is not a fast process and requires hard work, but it needs to be done. I have worked with secondary school children whose parents haven't addressed soiling and the cost to their self esteem and body image is huge.