For UK retailers it's hard to beat Heinnie Haynes they have quite a wide range of stock and their own exclusives in collaboration with manufacturers that alter popular models to be UK no-reason needed legal.
Direct retail tends to work Twisted Assisted and Hoo knives do well.
American retailers are a pain. Knifecentre have a high minimum order. BladeHQ just won't ship to the UK. Similar story for most. Even the giants.
Amazon UK seems to have only off-brand weird knives. Not like Sanrenmu, decent but obscure chinese knives. Just novelty tat in 3Cr and tinfoil. Amazon com won't ship knives to the UK.
Ebay uk has a strict no knives rule.
Retailers I've had some success with:
Knivesandtools.co.uk - this is a very limited subset of knivesandtools.com a Netherlands based retailer. The co.uk site has the knives they are willing to risk shipping to the UK.
Lamnia.com - the website doesn't give much info about the knives but if you know what you want, they probably have it and will ship it.
The untrained parcel pokers from Border Force may seize your knives but as long as what you ordered is legal a polite letter with a clearly stated willingness to go through with legal action will, in my experience get them to sheepishly send you your knives with an admission they aren't what they claimed (always a flick knife or gravity knife). If your foreign seller will accommodate, ask for any button or crossbar locks to have the pivot tightened so as not to drop shut (or open). You can loosen it later but your typical border force agent gets lots of training on drugs and half a day in a portakabin if they're lucky on knives and other bladed articles.