Lovely jacket! You can use flannel, but remember PLEASE wash your flannel in hot water and dry in dryer, before you cut out your garment pieces.
Living in Texas, I can't use a "batting" jacket. A few years ago I washed my quilt cotton for my jacket first, then placed the fabric (pressed) on the top of unwashed muslin and quilted the fabric together. Then I washed my fabric and dried in the dryer. (I was looking for a "quilted look" without any filler. It worked out great. After I washed the fabric, I cut out my jacket. I put a lining it in, but was not required.
Please post your finished jacket. Best wishes.