FWIW, "sunset western garden book" (the so-called "bible of western horticulture", LOL) recommends them only for their zones 16, 17 and 20-24---in other words the mildest near coastal areas of california and perhaps extreme southern oregon. they need excellent drainage, rather lean somewhat acid soil and protection from hard frosts (likely no lower than -3 c or so for brief periods) as OUTDOOR plants without protection. would guess that mildest near coastal areas of cornwall and devon (and the channel islands) would be optimal to try the plants. my guess is if you can grow metrosideros (new zealand christmas tree) or certain other protea relatives (banksia for example) in your area then you can possibly grow this species. cooler places would potentially need either periodic winter protection OR treating the plant as an indoor/outdoor plant growing in a container and moving indoors for winter protection. hope this is of some help.

I've heard from a considerable authority on the subject that they've been injured in a fairly recent winter at the Berkeley Botanical Garden in Northern California. I found it a little hard to believe at the time, as that is an area that seldom goes below -3C/28F and has been rated zn 9b at the worst. (current rating zn 10a) So I don't think they'd be happy in most of the climate of the British Isles, except perhaps Scilly, Penzance, Channel Islands, extreme SW Cork, etc. OTOH one should be careful about extrapolation from several botanical garden specimens to the species as a whole; temperature decrease faster with elevation in the Southern Hemisphere than in the northern and it's possible the species has enough of an altitudinal variation that some collections floating around the UK would be a bit hardier. I know it has been grown on Scilly, and have a very vague memory of reading that a garden along the Cantabrian or Asturian coast of Spain had some of them; of course that doesn't prove much as those areas are about as mild as Berkeley. The ones in the Bay Area almost certainly came from a Southern California wholesaler.

Found this...it is naturalizing on the Scilly Islands. But that is quite a bit milder than most of the rest of the UK.

Here is a link that might be useful: http://www.leadupthegardenpath.com/news/scilly-wild-flowers/

Sorry about that George! I had this open in a window as a single post, and came back to it about an hour later to reply. I see there is some overlap in our posts.
FWIW, I'd guess that some Metrosideros are a little bit hardier, and some other Proteaceae quite a bit hardier, even Banksias. I had a Banksia from Desert Northwest, B. canei, that survived a winter here on a south wall with no injury. It wasn't a very cold winter but it did go down to around 10F. Alas, the plant died in the spring because we had a stealthy spring drought and I think its location coaxed it into trying to grow too early when the soil was bone dry. I was concerned about overwatering it given that it was a Proteaceae, but I underwatered it! No doubt this winter would have killed it off completely though.
2 Grevilleas did survive to varying degrees, G. victoriae and G. juniperina.

I think the silver tree is quite tender because, IIRC, it comes from a very mild part of SA and its range doesn't extend very far inland.

thanks for your insights and info, David, there is indeed variation in cold hardiness among the banksias (no surprises i would think) and canei would likely be regarded as one of the hardier species (mine is looking so good right now but don't know what exactly might be the cause, cold or something else???)---thinking more about b. serrata or ericifolia as a possible indicator species for the leucodendron. that said, there is possibly room for variation according to plant provenance/seed source within most every type of plant and who knows what might be possible if there is a somewhat harder selection hiding somewhere. still, think we both might agree that the "silver tree' has a reputation for being demanding in its requirements in general let alone in its frost resistance and most parts of the u.k. (except for the parts already mentioned) may not be suitable for long term, out of doors, in the ground, unprotected, cultivation of this plant. my best guess is that if the O.P. can grow metrosideros out of doors they have a reasonably good shot with leucodendron but you can just as easily put in another "indicator" plant of relatively equivalent tenderness (LOL) as a baseline plant. hopefully, the O.P. will come back with some comments and info of his own at some point.

Hi, I was going to get some seeds for the Leucadendron Argenteum and attempt to grow them in london which is where i'm based.

I'm surprised that the Isles of Scilly would be milder than the rest of the UK considering its out in the sea there where surely it would be exposed to the elements, more winds and rain?

Areas immediately along the ocean and small islands are moderated by the ocean surrounding them. The ocean keeps these areas a bit warmer and a bit cooler because the ocean is so large and absorbs a lot of warmth.

Here is a link that might be useful: how the ocean moderates temperature