Unfortunately that attitude is very prevalent in many bee clubs. I usually advise people to just say that they are starting a Langstroth hive. The Flow hive is a Langstroth, just the super is different. The brood boxes should be managed the same way as your locals manage theirs, and we can help you here with honey extraction advice.
I think that Flow also has British National size Flow supers now - perhaps @Freebee2 or @Bianca can weigh in on that. You may need to buy the rest of the hive parts locally, if Flow doesn’t supply those, but if so, just come back here and I can help with describing what you will need.
The reason for mentioning National hives is that most beekeepers in the UK (and Ireland) use that size and style of hive. You will probably find it much easier to get a nucleus on British National frames than on Langstroth frames. However, even that is not hard to work around. One member of this forum developed a very neat way of adapting National frames into Langstroth frames to get the hive started. You will need to rotate the adapted frames out of the hive over time, but that isn’t hard. Here is the link:
You can do this. It isn’t easy, but it isn’t that hard either. With some determination, you will have a very rewarding hobby for years to come. 
By the way, late May is pretty late to start a hive. The main nectar flow is mostly over by July, so the bees will not have much time to build up. You may have some Autumn nectar too, it just depends on what local forage is available. Most nuclei are ordered the year before, and installed around April, depending on the Spring weather and queen mating timings.