Here is a list of retailers in the UK that sell the Denise set.
As for quality:
I've been using my Denise set for years without the slightest problem. BUT they're not to everyone's taste.
The part you knit with is made of resin; they feel firmer and more solid than most plastic needles, but if you don't like knitting with plastic you may not like these. They feel warm in the hands.
They come in US sizes 5-15. Obviously you will also need other needles if you use fine yarns.
The points tend to be rather blunt -- fine for most purposes except lace.
Some people find the cords thick; I find them perfectly fine.
They are quite light in weight.
The number of each needle is embossed on the surface; hard to read, but it's there.
The mechanism that holds the tips to the cord is very easy to use and very secure.
The parts come in a neat box with a place for every component.
KnitPicks Options are metal. They're slicker to knit with than Denise, but not as slick as Addis.
The points are sharper than Denise.
The cords are thin and very flexible.
They come in US sizes 4-11. You'll still need smallesr needles for socks, fine lace, etc.
The tips screw into place; you then have to tighten them with a little bent-pin sort of thing. When they were first introduced, a quality control problem caused many of the joints to fail, causing stitches to fall off the needle. I believe KnitPicks have improved the joins, though.
They are a bit heavier than Denise.
The needles are not marked for size. You have to use a needle gauge or mark them with little tags, which KnitPicks sells.
The packaging is a zip-up pouch with clear plastic envelopes for each needle.