It takes a very specific and unusual sort of bastard though to actively write his partner and children out of everything. The usual situation for unmarried couples will be that the house is co-owned (if there is a house), that there won't be any life insurance, that there won't be any will. In which case the surviving partner gets the house and the children get the inheritance.
For the house to be owned by one partner only as a default, the mortgage would need to be entirely in that person's name ie based on their income alone OR they would have to pay additional £££ to have a non-standard agreement drawn up. ie one person would have to actively seek that and the other would have to agree and sign the forms etc.
Pension beneficiaries are decided at the discretion of the trustees. People complete an expression of wish form but that can be over-ridden by the trustees. In a case where eg a man had a partner and children but had said he wanted his pension paid to his mother, the trustees would likely over-ride that. if there was a childrens element to the pension then the children would get that irrespective of what the person had said.
Life insurance I am not so clued up on
My point is that the default position for people who do nothing is that pensions and inheritance are probably going to be fine.
To make sure your partner and children got nothing, you would have to go out of your way, paying solicitors £££, to arrange it, and even then it might not work.
I accept that many men are financially abusive, you only need to look at these boards to see that. But I contend that it takes a very specific sort of wealthy and thorough bastard to actively write people out. I just don't believe that this is enough of a problem for the law to be changed in this way, given the impact that it will have on thousands of couples who are in casual relationships with no children, who for the reasons I have outlined above will be very unlikely to take action to change things back to how they were in the first place.