Zavvi are currently offering 2 Funko Pops (choose from 6 pages of Pops, there are a great many to pick from) for £18 however they are also doing a FREE mystery geek t-shirt if you spend £15 on Funko products, meaning if you pick 2 Pops and add in the mystery t-shirt into your basket you will get all 3 items for £18 (done it twice myself so far and will probably end up doing it again at least once more).
Pops selection - http://www.zavvi.com/merchandise/offers/pop-vinyl-multi-buy.list
Mystery t-shirt (sizes Small to XXXL in Mens and Small to XXL in Womens) - http://www.zavvi.com/merch-clothing/mystery-geek-t-shirt/11181315.html
P.S. I did get the same t-shirt twice so you all know but it was a pretty cool Thor one so didn't bother me that I got 2 of the same design and it may be because I went for XXXL, other sizes may have more variety.
No code needed, just add them to the basket and Zavvi does the rest during checkout.