Food in the uk that is labelled gluten free is allowed to contain up to 20 milligrams of gluten ( 20 parts per million of gluten in them) this is deemed low enough for people who have coeliacs disease.
but each person is different, both my wife and son are coeliacs and some things that my wife can enjoy without any problems make my little boy ill (he obviousley is a lot sensitive)
if you have a concerns when shopping for gluten free foods you can nuy a directory from coeliac uk
https://www.coeliac.org.uk/shop/products/78339/
which all the major supermarkets contribute to and there is a facebook group called "coeliacs in the uk" which is full of info and people are always willing to tell you if its safe or not, (just post a pic of the ingredients)
If you think you have coeliacs disease then its important to carry on eating gluten until your diagnosed because at the moment the only way of diagnosing this is from having an endoscopy from your intestine and you need to be eating gluten for this to show up :(