Hey, I work in UK airport security so I know a fair bit about UK knife laws.
* It is indeed illegal to sell a knife to anyone under in the UK. Also bear in mind we have rules regarding how a knife can be sold that I advise you to check in detail first. Can't put it on ebay for instance.
* It is indeed illegal to carry a locking knife without a good reason. Even a knife that meets the UK legal carry requirements could be confiscated if the Police believe it to be a threat, which is entirely left to their judgement. Ergo be careful where you carry even a legal blade, no pubs, bars etc.
* Buying locking knives is not illegal. Just carrying and selling them that can be iffy.
* If you use a legal carry knife in a threatening manner they will do you for possession of/assault with an offensive weapon. The laws here don't apply to the objects themselves rather the manner in which they are used. Hitting someone with a rolling pin could get you the same charge.
* You should be fine bringing in into the country in checked/hold baggage. I can't guarantee customs won't ask you about it but I'd say it's highly unlikely. Even if they do it isn't on your person and you have a reason to possess it so you should be ok. Even if they take it from you it's no great loss, you can always order another one while you're here.
Finally someone already mentioned the Lansky world legal as a good UK edc, and I'd second that opinion, have one myself. Might also want to look at the Spyderco PKUK as another option.
I know all the laws seem crazy restrictive to some people but don't worry about them too much, it's not as Orwellian as they first seem. As someone who lost friends to knife crime it was a real problem here and sadly I view the laws as a necessary evil.