Allowed by whom? If you've bought through a trading platform like ebay or amazon marketplace, then it's probably against their rules to review in exchange for any sort of payment or quid pro quo, but if your review isn't an obvious shill, who's to know?
Would you review anyway? Is £15 enough to sway what you write (as opposed to motivating you to write at all)? Where can you spend the gift card, and how long do you have to wait to get it?
If it's a platform where reviews are normal/expected, and I was going to do one anyway, I don't think I'd let the offer of a gift card put me off. But I don't think £15 is enough to get me to write anything substantive where I wouldn't otherwise bother, and it's certainly not enough to turn me into a shill.