"When it comes to figures plucked out of the air, the UK seems as prone to it as the EU."
Why not DMcG? It is the game that they started. So going by what is sauce for the goose is also sauce for the gander, no-one on the EU side of the fence can possibly object. What it might prompt them to do is have a stab at justifying their 100 billion.
Meanwhile as they witter on about the bill the clock ticks down and they are faced with getting nothing. We on the other hand by today's prices are automatically 11 billion better off every year after 29th March 2019. The EU and Michel Bernard Barnier can threaten all they like, when we leave, and if we leave without a deal, it will be the EU that will be decidedly the worse off - without our yearly ever increasing level of contribution and without our business (We buy more from them than they buy from us remember - we can always buy from elsewhere - who do they sell their new surplus to?).