Welcome to the forums. Most of us are US based, but we do have quite a few Canadians and some UK members who may be able to give you more local supplier or product recommendations.
Stateside, we typically call the middle layer as "batting". The choice of wadding/batting is personal and is one part what is right for the project and one part what is right for the person. I make usable/washable blankets, I have different concerns than someone making an art quilt. How you plan to quilt the projects is also key -- there are some things you are going to notice as a hand quilter that you might not notice using a long arm.
Right now, the "standard" I see is Warm & Natural, made by this company:
https://warmcompany.com/
I myself prefer less quilting than is currently popular and typically use a puffier poly batt. Is also a weight thing, as well as allergies. There are many choices and reasons for those choices.