My colony has drawn and filled about 8 of my 10 Langstroth frames in the single brood box they are in. Should I be adding another brood box or my Flow super first?
In the UK, this time of year is really too late for adding a super. If you need double brood boxes, I would add that instead. You can always take it off in September if they haven’t used it - that will help them have less empty space to heat over winter.
Second brood box with foundation would be my vote. Feed them if they are not getting any nectar so they can set up for winter. Once you get a hive through winter its a pretty tough beast but that first winter can be a killer.
Agree with Dawn. Flow is largely over in the UK and bees will be reluctant to draw wax now they are preparing for winter.
I would do a serious inspection about the need for a second brood box so that the hive can build up numbers to survive the coming winter. Adding a honey super at this time would not be a wise choice. Your aim should be to get the hive strong enough to last over winter then you will be in front for next spring. Warmth and stores now is what you aim for in the hive.