One aspect of EU-UK relations that will not make it into (any) future partnership agreement is cooperation on foreign, security and defence policy.
I explored why there have been no negotiations in this area in early July via @chathamhouse expert comment
Why the UK Has Taken Foreign Policy Out of Brexit Negotiations via @chathamhouse https://www.chathamhouse.org/2020/07/why-uk-has-taken-foreign-policy-out-brexit-negotiations?CMP=share_btn_tw
Why the UK Has Taken Foreign Policy Out of Brexit Negotiations via @chathamhouse https://www.chathamhouse.org/2020/07/why-uk-has-taken-foreign-policy-out-brexit-negotiations?CMP=share_btn_tw
And also had a chance to discuss costs/benefits with @CommonsFREU oral evidence session https://committees.parliament.uk/oralevidence/751/html/
Without an EU-UK formal agreement my view is that there will be a period of 'mudding through' - see my piece 'Missing in action: The EU–UK foreign, security and defence policy relationship after Brexit' for @MartensCentre https://journals.sagepub.com/doi/full/10.1177/1781685820975877