I know a mortgage is calculated on your earnings... but what if you have a really low basic wage, but earn a lot on commission? Is this taken in to consideration?
Depends on the lender. A lot will not take into account anything other than basic while there are many that will let you certify a total income without atating how that is made up and others will let you count 50% or even 100% of commission. Best advice is to go and see a mortgage broker. They usually get their commission from the lender and will be able to get you the best deal and present your case in the best way. Find one that a friend or member of your family can recommend and trust the recommendation.
I think you might be looking for a Self-Certificating mortgage. My sister and her boyfriend got one of those because he's a musician with fluctuating income and was finding it hard to get a traditional mortgage. Your mortgage advisor whould be able to advise you - just a word of warning, though. Certificated mortgages usually have a higher rate of interest (at least at first), because it's riskier for the lender.
You can get a normal (no self cert) mortgage from quite a few of the main lenders so long as you can show a consistant level of commission.
My previous job was fairly commssion heavy (I doubled my basic salary with commssion) and by providing my P60 for the preceding two years, the bank happily gave me a mortgage based on that average earnings.