As EU negotiations will be very protracted possibly 2 years or more, therefore I do not want to shoot all my ammo in one go so here's my first shot:
Apart from the UK making the second largest contribution to the EU budget fund, we also pay 60 Billion USD into NATO for the defence of Europe, the largest contribution by far of any European state with France next at 40 Billion USD.
Europe will not exist in its present configuration without the security guarantee that Nato provides, the Britex negotiations need to take cognisance of this and make the final settlement a trade and security package. It is worth noting that without the UK in Nato the US and Canada may also want to consider their positions. ( hard nose negotiating position yes but may be necessary)