The 15% sign-up voucher only works on full-priced items. If you add sale pieces or clearance items to your basket, the code will simply not apply a discount to them. It is generally best saved for new-season stock that is unlikely to be reduced anytime soon.

It definitely excludes anything already discounted. Also bear in mind Whistles only lets you enter one promotional code per checkout, so you cannot stack it with other vouchers either. If the item you want is already on sale, the markdown price is almost always the better option anyway.