In reply to clouds:
Kind of had the same problem, but being size 6 to 6.5 I had at least *some* choice. Although most boots I found where 7 and upwards.
Don't forget that you want to wear some thick socks in those boots, plus our feet swell at altitude, especially high altitude of 5000m and above. So if you're normally UK 4, you definately wouldn't want to buy a UK 4 boot.
I went to the showroom of my expedition organizer of choice and got an outstanding service in choosing and fitting the right boot. From measuring my feet sitting and standing (and the difference between), advice and on socks etc and trying on a few boots. I've got bad circulation so anything other than double boots were out of the question.
I finally settled for the already mentioned Boreal G1 Lite. Intend to break them in in Scotland this winter and then use on the likes of Elbrus, Island Peak, Chimborazo etc. I ended up buying size 8 (!!!) wearing the boots and mountaineering socks for about an hour while we talked about other gear. Could still wiggle my toes a little bit, but heel lift was minimal.
Can only recommend to go to a shop and try them on with all of the above in mind. I would never buy them online really. At 6000m I don't want to worry about the fit of my boot, blisters, numb/cold toes etc. The new Phantom 6000 and G2 are certainly options as well.