When you claim, give them your estimate for the current years income. They ignore the first £2k drop (so if it's dropped from £20k to £10k, they'll assess you as earning £12k) and you would then get paid from the date of your claim (I think they back date 6-8 weeks?).
It's usually based on prior year income, but they will consider current year income if circumstances have changed by more than £2k. It also depends on who you get on the phone - some are nice, some are awful! They do base the income on what's earnt between 06/04/15 and 05/04/16 and so on though so if you've had full time wage 6-7 months through the year, your income may be too high xx