if uk customs thinks it isn't a gift, they will charge the recipient vat+duty.
How would they ever find out? The only situation that comes to my mind would be if a copy of the payment/invoice is included with the package.
I've recieved quite a lot of international shipments, 90% of the time they just were underdeclared and nothing happened. Just don't make it too obvious and inform yourself which values are free of tax in the country you ship to. If you mark it as a gift you don't need to send the buyer a declaration or anything, at least not in germany. If you ship anything that is underdeclared NEVER include the original packaging, they will always think it is a NIB item (except when severe wear is visible or something like that). The chances that a package gets opened up and checked are pretty slim, but if it happens they will just google the product and charge the recipient what they think is appropriate(sometimes also a fine for tax evasion).
I ordered something RC-Car related back in 2010, which ended in my worst experience that I had with customs ever. The item did cost around 330€ in germany, and there were basically no shops offering it. The thing is, back then the item did only cost $250 shipped in the US, the exchange rate was also very good so I think it was around 170-180€, a huge difference in price. Well, it was declared for the amount I paid, but it was opened in a "routine check" and they googled it to fing out how much it costs. At that difference they thought the copy of the invoice was faked and in the end the whole thing did cost me an extra 100€ at the customs office -.-