Typed all this once after work then lost it in a system time out....
Russian/former Russian sites:
Baikonur/Tyuratam : 46 deg North
Kapustin Yar: 48 deg North
Vostochny : 51 deg North (New facility in the Russian Far East to replace Baikonur, which is in now-independent Khazakstan)
Other sites:
Woomera : 31 deg South
French Guiana : 5 deg North
Kennedy : 28 deg North
High latitude sites:
Kodiak : 57 deg North
Plesetsk : 63 deg North
Although the southernmost points in Canada are at about 41 deg North, they're in the Ontario "salient" so any space launches from there would have to cross the north eastern USA, creating safety and political issues. Most of the practical launch sites in Canada are well north of it's 49 deg southern border, which is why, when looking for a "Commonwealth" launch site, Woomera was the clear winner.
You can launch rockets from northern sites, as Plesetsk and Kodiak demonstrate, but they are mostly used for high-inclination polar orbits and can't be used economically for equatorial or geo-stationary ones, which is where most of the commercial revenue lies. Polar orbits are mostly of interest for scientific and military purposes, so their rewards are not financial in nature.
Even with their sites being further south than most theoretical Canadian ones, the Russians still pay a price for their geography, getting less payload per pound of rocket than launches from more equatorial sites. Since any large-scale British launch programme requires a stretch of the imagination and wallet, it's hard to see it volutarily accepting the extra penalties of a Canadian launch site when Woomera was available. The running costs of a large space programme (i.e. expended rockets) would be much more of a burden on a limited budget, so the pressure would be on to develop the most efficent launch site as soon as possible, even if that meant investing in exotic options like sea or air launch.
