And another thing - the UK is a pain in that in some countries it doesn't uprate the pension each year with inflation, but in other countries it does.
If you retire in Canada your UK State pension will stay the same forever. If you retire in the US, it will change every year just as if you lived in the UK. So if you're planning on buying those extra years - just bear in mind depending on the country of residence it may not end up as much as you think.
Also, Canada withholds 25% of CPP, OAS, RRSP withdrawals in most countries. There is a list of which they do and which they don't - if you retire in the UK for example (despite not having a social security agreement!!!), Canada withholds nothing. If you retire in France, they keep 25% and that isn't recoverable except as a credit against your French taxes!
It is, in short, a bloody mess.