请问如果在英国西斯罗机场安检后候机时购买的

如果你是安检后已经办理出关后的，都是免税店。也就是里面的商品是已经扣除税金了的。
一般结账的时候，给你的包装袋都会有DUTY FREE的字样。

Indeed, you see they are called 'duty free' shops
but that does not mean they really sell in cheaper prices。 They are duty free, but they are also free to set their prices, right?

是的 那些地方的都是扣除税费的

英国没遇到过免税店还能再退税的，你问问店员

出关之后的是免税商品

http://www.globalblue.cn/customer-services/tax-free-shopping/tax-free-shopping-faqs/