The terms we are using here are from Barb T.'s Book "The Art of Kiltmaking". The Stabilizer is a length of Cotton Broadcloth similar to bed sheeting.
The Interfacing is "Hair Canvas". I'm not sure what the British term is but it is not canvas in the sense of tent fabric but a specific tailors product made from horse hair or synthetic. It is stiffer in one direction than in the other. This allows it to bend around the waist but not develop puckers in the up and down direction.
Both of these are readily available but you have to be specific in nomenclature when asking for it.
Perhaps one of our UK members can come up with the proper British nomenclature for you.