We are blessed, if not in climate then at least in our gold leaf supplies. Listed below are my favourites. I or my very demanding designer sister have used them all. Disclosure: Amazon likes me enough to offer me a commission on sales through this site. In turn I don't hesitate to recommend the Big A: they consistently offer a fab service.
That said, the Amazon UK search function is slightly eccentric, omitting a perfectly good genuine silver leaf because it’s from a different department, or including a glass teapot among the gold leaf searches. (I don’t know why.) So you may prefer to search third-parties direct: Wrights of Lymm, A S Handover, Gold Leaf Supplies.
Another reliably great online supplier of gold leaf: lots of variety, high quality, prompt delivery, and a dedicated page for sale items. Not cheap, and their postage starts high, but very good. One of my go-to sites when Amazon are feeling off-colour. Also stocks gold and silver roll, and all the associated paraphernalia you can imagine you might need for any gilding job, and gives useful online advice. And they stock pink silver leaf. Yup. Pink.
They pay me nothing to say all this :-)
Kremer Pigmente: based in Germany with good English and some European languages spoken. Gold and other metal leaf sold and posted at reasonable rates. The invoicing system is old-fashioned but be warned, you will care nothing about that mild hassle when you see the range of beautiful traditional art materials they stock. Indigo lumps! Cochineal beetles! Orpiment!
Ken Bromley Art Supplies is a friendly, helpful Bradford-based fine-arts supplier with a good line in items like spotter brushes and variegated metal leaf that are not always so easily found. Strong website, easy to use and buy from, and good prices. Also check whether the Bromley product you want is on Amazon with free delivery.
Wrights of Lymm is one of the oldest surviving manufacturers of gold leaf in England, with lots of names to drop and twenty-two categories – not products – categories of metal leaf products on their site. Good quality, high prestige, and several of their bestsellers usually available through Amazon. Yay! My sister uses them and recommended them to me recently, and she has very exacting standards, so I think you'll be all right with Wrights. They supplied the gold leaf used by the 2009 Turner Prize winner, which is almost as good a recommendation as my sister's.
It’s a pity to buy from Cornelissen online (look at the left-hand menu) because going to the real place just up from the British Museum on Great Russell St is like visiting a cross between a wandmaker, an apothecary and an old-fashioned tool-shop. I have never ever walked out with an unbruised bank balance. Cornelissen stocks different types of real gold leaf, imitation gold leaf, and various types of other metal leaf (silver, palladium, aluminium, etc) too. Worth a visit just to see the beautiful old wooden drawers all up the wall behind the counter.