Thinking here, there's more than one way of confirming your annual salary of course. Yes one method they use is to take 6 months and make that up to an "in theory" 12, however the other way is to state you are using as evidence pay in the last 12 month period.
For that, I think you provide 12 months worth of payslips (Yes it's every 28 days, but use 12 calendar months and I'm certain this would be fine, and if not, then it clearly says this would not be grounds for refusal, but they would request an extra payslip or two), along with your P60 which although not essential, can't do any harm (As long as its over £18,600), and this way you should meet the requirement.

I don't think they use the last 6 months wageslips to make some sort of projected income, if you are submitting slips for the last years earnings option.