If you have good health and are not exposed to any dangers in your work or lifestyle then the standard rate may apply. Insurers however can apply loadings to those rates if you experience medical difficulties or are exposed to other dangers. These are calculated in the same way as your initial premium. However the insurance companies can change their underlying rates in future and there is no guarantee that the rates applying now will apply in future years. There are many ways in which this can happen.
Most companies also offer a discount for larger sums assured.
If your cover is indexed to inflation which, in a future year, takes you into a new discount bracket, then your premium is likely to reduce. Most companies also offer a loyalty discount when you have retained your policy for a specified period. This may result in a reduction in your premium. There is no magic formula. Only you can decide what you would want your family to have.