Welcome to the Flow forum! I am afraid that I am going to be a bit infuriating, and not give you a straight answer, but hopefully you may find some of this useful. 
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Have you thought about adapting the national frames to the flow hive, rather than adapting the box to the frames? Here is a brilliant post telling you how to do that very easily:
Convert UK national brood frame to langstroth in 1 minute
If you do this modification, you don’t have to hurry to cycle the frames out of the brood box, and inspections will become a whole lot easier.
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If you do cycle the frames out, I would just do one at a time, but I really think that this is not the time to do it. The best time to start would be next spring, when several frames will likely be fairly empty and you won’t be removing a lot of precious resources.
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I used to run WBC hives in the UK (also with National frames). One brood box was not enough to overwinter in Oxfordshire, and I would imagine that to be even more the case in Lancashire. We used either jumbo brood boxes, or brood and a half. To keep things simple, I would counsel you to consider a second deep Langstroth brood box before filling the super. If you don’t have more brood space, you will have to feed them very attentively all winter and into spring, or they may well starve.
Please ask more questions if you think we can help.