What they ARE saying in effect is that we must drop the little-Englander Tory hubris and negotiate sensibly about what we need to pay on exit.
But the UK has shown it is prepared to negotiate sensibly.
"Sensible" doesn't mean rolling over and agreeing to pay a bill for which no justification has been provided. The EU appear to have plucked a figure 'out of thin air' which they believe is enough to give them an upper hand in negotiations, and compensate them for losing one of the biggest contributors to their little club.
If the figure hasn't been 'plucked from thin air' then surely they should be able to show how they arrived at the figure.
Not a lot to ask, is it?