I've been shopping a fair bit lately and, despite what I thought when I went out, I found there actually was consistency between shops.
In jumpers I took a size 8 in Wallis, Marks and Spencer, BHS and Principles.
I bought size 10 trousers in BHS, Next and Principles.
I bought coats in a size 10 from Betty Jackson Black, Principles and Jane Norman but the wool jacket I got from M&S in a 10 had to be returned for an 8. I also had an 8 in a cardie and in a skirt from M&S.
I bought size 10 jogging bottoms in both BHS and Bon Marche (and both are roomy).
I bought two pairs of Jeans though that were both from McKay & Co and had to have a 10 in one pair and a 12 in the other (not different styles either).
So, as I said, I was surprised that I generally did take the same size in different shops as I'd never found that to be the case before but it does look like M&S come up bigger than others.
If you were to see me you would know I'm not really a size 8 and I can't get my head around being that size in the shops - I feel that I look like a 16. I'm so used to just holding something up and knowing my size but I've just got no idea at the mo.