Or as I understand, you can also have been paid £18,600 in the last year, and it doesn't have to be continuous - for example (because I'm not putting that clearly), say you earned 17,000 in the last year, but then this last month gone, you managed to earn £2000 overtime (yes I know that's ridiculous, but it's just an example to make it easy to understand). Well that means if you counted back, you will have earned over the £18,600 in the last year, so it would count.
If say you got promoted or a new job though, which in theory will pay over £18,600, then you only need to earn for 6 months proof - they will work it out that you'll get the amount needed over the rest of the year.
I hate to say it, but in both these cases, it's good of them, because they could have said you must always wait and have the proof on the yearly P60.
Not that I agree one little bit with this income requirement, but at least they are being more fair than they could have been. Hrmph!