mortgage insurance term

Roughly yes - it means that the policy will run for 25 years. You can cancel at any time, though there may be fees to pay if you cancel in the first few years, to reflect the cost of setting up the policy and any broker's commision.

Life/critical illness cover is not like car insurance or home insurance - you do not renew every year and the expectation is that your policy will last the full term. This is a good thing - it's a long term policy, and if you were renewing annually you would NOT want to be having to renew or find a new policy while you were having treatment for, say, cancer. In return, you have no duty to disclose changes to your health every year, and your premiums are fixed for the duration of the policy (unless you opt for reviewable premiums, in which case they can increase in line with changes in the market as a whole but NOT as a result of your own health or other personal circumstances.

So there's no real concept of doing an annual shop around or changing insurers on a regular basis. Certainly there's no harm in looking around every few years to see if you can get cover more cheaply, but all else being equal life insurance gets more expensive as you get older so finding cheaper options in future would be the exception rather than the rule.