There are two ways to do this.
Ask the CAA by email, explain the situation - licensed balloon pilot fully involved, sub 250gram UAV, non commercial, what separtion required? The guys at the CAA tend to be interested in aviation and I have always found them looking to help within the bounds of aviation regulations. Print out the email you get back just in case any thirf party who thinks he knows better gets on you case.
Or two - you are both consenting adults, go ahead and do it and apologise laier if anyone is offended.
The only thing that concerns me is launch and recovery of the drone, how easy is it going to be to hand launch / catch in amongst the basket suspension cables? Would it be sensible to have a lightweight platform - like a lollipop man;s “Stop Children” sign with a rim for the Mini to land on or will thayt bring its own hazards?