Hi
If you are uploading a video, it needs to go onto the likes of Youtube or Vimeo and then a link posted here.
Is it just her particular stance when you took the photo or is she twisting her neck/head to the side? Is she still eating and drinking well?
I'm also in the UK and there is a wonderful service for doing faecal floats called Westgate Labs who offer a mail order service which is very fast and efficient and the staff are really knowledgeable and helpful and best of all it is not as expensive as a vet to do this testing. It is about £15 for a poultry sample but that includes sampling kit and first class postage and if you can get it in the post before lunchtime, you often get next day results. I have used the service for my horses for years for worm egg counts and can highly recommend them. Kits can be purchased direct from them or via other online sites or sometimes feed stores will stock them.
I would not rule out Marek's just because others are not displaying any symptoms. Marek's is odd in that it has dormant phases and outbreaks occur at times of stress. What stresses one bird may not stress another. It may be something as simple as recently been knocked down the pecking order. Just like people with cold sores, not everyone with the virus has an outbreak at the same time.... it is the same family of virus ie Herpes.