1) the UK has accepted that these costs are outstanding liabilities due to prior agreements the UK has also entered into, and 2) the UK has accepted in principle that it has obligations here, and 3) that the UK has even accepted a figure for it.
The figure thing is a negotiation. But going back to 1), liabilities for what exactly?
When you pay something as part of a deal, it is in return for something else. What exactly would the UK be receiving in return from the EU if it just left with no deal?
The payment is in return for some kind of deal for the future, that much has always been obvious. If pensions were problem the solution is quite straightforward: the UK takes over those obligations towards british citizens, no transfers necessary.
The correct way to see this is that the UK has been contributing to the budget of the EU and is a co-owner of its institutions. If anything it should be the EU "buying out" the UK's interest in the assets of the EU: buildings, funds, institutional knowledge, trade deals, etc. But because the UK is leaving on its own and no treaty clause provides for such payment, it can't demand that. However, the opposite is also most definitely true:
the EU has no right to demand anything from any member state that chooses to leave. And this is an issue far greater than brexit, and one that I worry about what kind of precedent is being set here. What is actually happening is that the EU, because it is "larger", is shaking up the UK for whatever it can get.
Moreover, I must say that I find it disgusting that many self-styled leftists who claim to be "anti-imperialists" or whatever keep cheering for the empire in this one, just because they dislike the current government of the smaller state being squeezed, and unmindful of the kind of precedent being set. Coherence be damned.
This is the same kind of crap that colonial powers did to their colonies negotiating for independence. The EU was supposed to be better, huh?