You've received a faulty product, send it back and explain that you need a working one ASAP because it's a gift. You've got 11 days before the birthday, if a company in the UK can't sort out a product swap in nearly 2 weeks they don't deserve to be trading at all.
I see this all the time with airsoft shit, people buy broken guns for new prices from retailers that offer a warranty, then spend their own money fixing them?! What the actual fuck, if you bought a TV and it was broken you wouldn't take the back off and start changing out circuit boards, you'd send it back.