With a “Guaranteed” policy the Life Company guarantees that it will never increase the premium.
With a “Reviewable” policy the insurance company reviews your premium level regularly (every 2 to 5 years but the reviewing interval can vary between Life Companies) and the company reserves right to increase your premium on the review date.
As you would expect, a “Reviewable” policy will start with a lower premium than an equivalent “Guaranteed” policy. However, the price of “Reviewable” policies will soon catch up and in the longer term are generally more expensive.
Some Life Companies have stopped offering “Guaranteed” rates on life insurance policies that include Critical Illness cover. This is because they experienced a higher claim rate than they originally expected. However, if you can find a “Guaranteed” life and critical illness insurance policy, you should seriously consider it.