For some reason I was under the impression that we too have to specify the lowest salary during the 6 months period. Your explanation makes sense though, I could just be imagining things
I don't think so, but if it asks for it on the application form, then put both salaries.
Basically, when it comes to calculating your past income, they can only consider the lowest level of pay during the 6 months (the lowest payslip)... which may not even have anything to do with your salary changing.
For example, if your salary was the same for the entire 6 months, but for some reason you got paid less in one month than in the other months (maybe you worked overtime in all the months except one, or took some unpaid leave that month, or were overpaid in a previous month and it was deducted the next month), they will only use the lower amount earned in that one month to work out your total annual income.
Or, alternatively, say your base salary was £1,800/month (£21,600/year), but actually you worked overtime in every single month, and your lowest payslip was £2,000, they would use £2,000 to work out your past income, not £1,800... so even though your salary is £21,600 on your application form, because you worked overtime they would actually work out a higher figure for the past 6 months... though overtime cannot count towards future income (unless it's contracted/guaranteed in writing), so they would still use the £21,600 figure for your annual income going forward.
So, you would still put your normal salary on the form, it's just that the figure they actually calculate from your payslips may be a little different to that number.