Keeping in mind that I live in the USA:
I purchased an inexpensive, light colored polyester blanket for a queen size quilt and used it for batting. The quilt came out gorgeous, heavy and very warm. A piece of fleece can be used for both backing and batting.
Backings: look for flat sheets in good condition at thrift stores and use them! I have several waiting for me to make a quilt for them! I also buy flannel sheet sets at end of season sales - price cuts of up to 80% if you can time the sales just right. You can cut the fitted part of the fitted sheet off and use it for piecing tops, and you still have a nice sized piece of flannel for backing. Plus you have a pillow case that matches the quilt.
A good sized piece of fleece or a fleece blanket can be used for both backing and batting, depending on the quilt.
Use your leftover scraps and make a pieced backing. You'll be surprised how good that can look with just a bit of planning.