When I asked a mortgage broker friend (Canadian) about this not too long ago, it was explained to me that yes you could lock in a rate for the "term" - but the "term" was 5 years, and after 5 years it would be adjusted depending on the prime rate, etc..... so from what I gathered you coudln't actually just get a plain old "25 years, x%" mortgage.
I'm not sure about other Commonwealth countries, but Canada definitely does not have this option. Our mortgages have a specific mortgage rate term (usually 3-5 years), separate from the life-time of the mortgage (usually 20-30 years; the 35 was/will be no longer be available). This means that interest rate fluctuations are more devastating when they rise for those who put little to no money down on their expensive housing purchases in Canada recently.
Amortization length of a mortgage can be up to 35 years for residential mortgages. 35 year amortizations were only eliminated for CMHC/insured mortgages.
I would contend that the system is more stable when borrowers are forced to consider the consequences of future interest rate increases.