The vast majority of mortgage payment protection plans can only payout for a maximum of 12 months. There are, however, a very small number of plans that could payout for a maximum duration of 24 months.
The usual limit of 12 months is far more of a concern for the accident and sickness section of the plan compared to the unemployment section of the plan.
It is normally the case that another job can be found within a year if someone is made redundant, even if that new job is not as well paid. The point is that you are still fit and healthy enough to work.
However, on the accident and sickness side it is possible that you are too unwell to work for many years for a more serious medical condition. This is something that will be out of your control and therefore only having 12 months of protection leaves a great deal of long-term risk.
Some insurers have introduced long-term Mortgage Payment Protection plans that can pay out for a maximum duration of 12 months for redundancy but can pay out for the entire length of your mortgage loan for accident and sickness.
On the accident and sickness side, these plans would continue to pay out either until you are able to return to work or you reach the end of the policy term, which is usually set equal to the length of your mortgage.
These plans are essentially Income Protection policies with added unemployment cover. For more information please see this page: Long-Term Mortgage Protection.