I was offered a promotion that I'm doing as a trial basis for a couple of weeks. I don't think I'm going to stick with it. If I'm paid at a
slightly higher rate for this ~two week period then go back to my usual hourly rate will that affect me in any way?
Nope, it won't affect you.
As you are non-salaried (paid hourly), your previous 6 months of income will be calculated as follows:
All 6 months of pre-tax payslips added together / 6 = average monthly wage
Average monthly wage x 12 = average annual wage
As long as the calculated average annual wage = £18,600 or more, you will meet the requirement, regardless of what hourly wage you were on during the 6 months.
For the employer letter, assuming you go back to your usual wage, they will need to mention that you are NON-SALARIED (this is very important because it will affect how they calculate your income) and they will need to state your usual hourly wage as your current income. If you like, the letter can mention the temporary promotion, but it won't have any bearing on the application as you will not be earning that amount going forward.