It depends on how M&S or JL treat credit card payments in their bureaux de change. I can't imagine that they would put them through as purchases though I might be wrong. I would think it will count as a cash withdrawal, attracting a fee, interest and no points.
If you use Curve it may get declined based on how the Creation computer is feeling. Or only transactions below £300 or £700 may be approved (there seems to be an individualised soft limit for Curve-Creation transactions).
If you are thinking of buying foreign currency from one of these chain stores, you might as well get it from Debenhams if you have anything to buy there. You get a £5 off £20 voucher when buying over £50, so you can split a large transaction into several smaller ones and get multiple vouchers. Buy more than you actually need, then you get 20% extra when you convert foreign currency to pounds (in the form of a Debenhams gift card).