This is the most sensible post on here.
Except for the UC. Your own house doesn't count as assets, and that's all she'll have. So she can just go on benefits and have to work as soon as they make you (is it something like 15hrs minimum when the kid turns 3?) and live off that and his child maintenance payments.
Equally she won't pay much for childcare. For every £1000 she "spends" on UC, she only has to actually pay £150. Of the 3 kids, ones in FTE. Ones about to start FTE, and the baby gets free nursery hours in a few months. Half terms and school hols she's going to have to pay for childcare, but again, at 15% of the normal rate.
She can be pretty sedentary for about 3yrs and get a little house out of it. But then it's going to be a bit of a wake up call. She's got 12yrs of maintenance for 3 kids, then another couple of years falling down to 2 kids, then another couple with just 1 kid. Assuming Dad never loses his job, gets a lower paid job, or sets up on his own and fiddles the figures so she gets £50 a month.
£260k is a good salary, but not with three kids, someone who doesn't work and this ridiculous lifestyle they both seem to chase after and can't actually afford. Let's not forget dad needs to be housed out of this little pot too...