There are many mortgage options available in the market, differing in
- Whether they are tracker or fixed
- Spread above the base rate (for trackers)
- Rate (for fixed rate)
- Length of "teaser period" and associated rate/spread
- Whether you can offset against savings or not
- Pre-payment options and fees
- Whether or not they are transferable to a new property
- Set-up fees
As well as this, assumptions about what interest rates will do in the future are important (if you think rates will rise it could be better to fix at a lower rate). Your own tax and savings situation also factors in - if you pay a high rate of tax on interest income, it may be better to offset your mortgage interest rather than earn interest in a bank account, but this tax advantage is not present if you do not pay a high rate of tax, or if you do not have a lot of savings.
Most mortgage comparison sites work by comparing either initial monthly payments or APRs, assuming (a) that interest rates will not change and (b) that you will not offset any of your mortage. They also don't generally take your individual tax or savings situation into account.
I could build a complicated simulator which allows for different interest rate outcomes, offsetting plans, tax rates etc but it feels like something like this should already exist. How should a layperson deal with this complexity?