There is a potty training section on MN www.mumsnet.com/Talk/potty_training but I think in your case it is more a neurological issue.
I had something similar with my son 4 years ago and I second what a previous poster has said. You need to disimpact, aka completely empty the bowel, if you want any chance of success.
We were seen by a gastroenterologist and they did an abdominal X-rays . My son looked like a drug mule. He had hundred balls of poo everywhere, up to his lungs. Some of this poo, was extremely old, and current poo would make its way around it. More often than not, only the liquid part could go and this is when he would soil himself.
The doctor explained to me that there are nerves on the bowel and when they expand, it sends the signal to the brain it needs to be emptied, but in my son's case and I suspect in your DD, if the bowel is constantly enlarged, no signal is sent, and no matter how much you talk/yell to your child, she can't help it.
You need to reeducate the bowel , to reeducate the nerves, and to do so, the bowel needs to come back to its normal size.
You need more movicol and to address the diet. My son was banned dairy for as long as he was soiling.
And he wouldn't notice he was dirty. He wouldn't notice the smell or the discomfort of having poo in his underwear. He genuinely didn't realise, which drove me crazy. He was 7.
Would your GP send you for an abdominal X-rays? We were in Spain at that time, so not sure how it works in UK. Because if she has the same rock-hard balls of poo, they won't come out on their own and not with a 2 sachets/day of movicol. You need guidance on disimpactation, diet, drinks ( a lot more water) and a lot of patience.
However, if she also has bladder control, maybe a specialist appointment, an urologist, neurologist is in order ? If it is a nerve dysfunction, you need intervention on exercise, schedule , again diet.
Read this paper www.ncbi.nlm.nih.gov/pmc/articles/PMC5332240/ and see if the recommendations you were given are on the same line.
If what you were told is incomplete, for example no physio, go back to GP and become that mother, a massive PITA.
Meanwhile, she needs to drink a lot of water, for both bladder and bowel and maybe she doesn't drink enough to avoid wetting herself, but it is actually making things worse.
Good luck, it is very stressing but she is suffering far more than you imagine.
Has something new happened? New school, house, sibling, divorce, ....