Ok, here is the scenario.
Germany moves 2 subs off the coast of UK to hit a convoy route. UK has no DD in range to conduct combat with them during UK’s combat phase.
UK purchased a DD and places it on the convoy route. That sea zone now becomes a contested sea zone…
Germany rolls 4 d6 on convoy disruption on the UK collect income phase.
Roll over to next turn
Germany at the start of conduct combat phase has a choice to make since they have a contested sea zone.
#1 during combat move phase they take the 2 subs and move them away. (retreat)
#2 stay in the contested sea zone and attack the DD with the subs and any other assets they have in range (air or sea) and of course UK could scramble air assets in defense.