Not all nationals end up on double brood ...I use 14 x 12 and I never need double brood. Plus, if you use a dummy frame at the end that's easy to get out and then inspections for me are done by just sliding the frames along to where the brood starts and I just inspect for swarm cells, disease and stores.
You may find, in the UK and where you intend beekeeping that, like me in a semi-urban environment, your flows are not as heavy as you experienced in Germany and are more steady across the season. You may find that, with the Dadant format, you end up with a lot of honey in the brood frames and less in the supers. Dadant brood frames are going to be a bit of a challenge for most reasonably priced extractors.
Whilst I recognise your enthusiasm for Dadants and their lesser cost in Germany I would agree with others who have said there are more common hive formats in the UK (bought in the sales and as seconds they are much cheaper) and if/when your beekeeping expands there are more options for extra kit and bees.
As always, beekeeping is about making decisions and the decision that you make is the right one ...but with the caveat that you consider and appraise the advantages and disadvantages. A simple chart comparing For and Against or even better - a SWOT chart - Strengths, Weaknesses, Opportunities, Threats which, if you do it honestly, may help your decision.
