Severe flat head syndrome, please help??!!!

My 6 month old girl has flat head since she was a newborn. She gets a good amount of tummy time and rolls. We’ve been using a flat head pillow for months no improvement. We went to a private specialist doctor as the NHS just REJECTED us, he said she’ll need a custom made helmet for £2,000. Can anybody please give me advice on what to do, I’ve heard that repositioning techniques don’t work after 6 months… will her head shape not improve AT ALL without a helmet??? :( :(