Splitting up ... how does it work financially?

WillowCat

£800 per month UC is too much based on those earnings.
I calculate (assuming here that neither you nor the children are disabled, the eldest born before April 2017 and that you are over 25 years old)
Single element   £409.89
Child element     £517.08
Total before deductions £926.97
Earnings (after tax, ni and pension)  £1,400.00
Deduction for earnings         £559.44 (first £512 of earnings disregarded, rest of earnings at 63p in the pound)
UC Payable          £367.53
The single element is due to reduce by £86 in April - though campaigns are running to get this extended.

Mrsn

Op if you are planning on going into rented accommodation then UC will be able to assist with this. £800 unless rent is included is highly unlikely to be an accurate figure I’m afraid or did you calculate some childcare costs in with that figure as you would be allowed to claim up to 85% of those if required also.