Can you recommend the best places where I can buy customizable boxes for my private label products in the UK?

I’m looking for options that allow me to customize the color, brand, etc.

It all depends on what you expect from the box and quantity - if you looking for luxury type like on your picture with filling inside you definitely should look into sourcing from China. UK company won’t bite the price but bear in mind you will fit only approximately 400 on a pallet so it will be worth ordering a full container where the unit price + shipping is below 1£. You have to add 3pl storage cost and get a supply chain in place but in the long term, it’s your only solution. All the big players have their boxes from China - on a previous job we were using 180k of them monthly

Take a look at Belmontpackaging

An alternative if suitable bro, Kite packaging do custom bubble mailers now you may want to look into the holographic ones. I believe this may reduce prep costs since it’s like packing a poly bag not folding a box.