The law calls that a complex lottery. But it's still a lottery, and the UK law on lotteries is very simple: No for-profit lotteries.

You can (and at least two big companies do) make money from operating a lottery on behalf of a charity, but you can't run a lottery where the profits themselves go somewhere other than a "good cause" such as a charity. And the UK has tighter regulation than the US of charities so it's harder to have a "charity" that basically just puts money into your own pockets.

Wagers on things which are part-skill and part-chance are covered by the definition of "gaming", which is one of the other forms of gambling in this law and seems like it was originally designed for games like roulette, poker etc. If the loot-box is designated gaming then the restrictions are no less severe and certain exemptions around lotteries wouldn't apply (so probably more severe).