Hello and welcome to the Flow forum! 
There are plenty of beekeepers in the UK with Flow hives who are absolutely successful with them. I haven’t seen them active on this forum recently, but @Paras uses a Flow hive, and @HappyHibee works for Landrover/Jaguar UK and runs their Flow hives very successfully at various locations in the UK:
Traditional beekeepers are often prejudiced against Flow hives, so I suggest you just say that you are using a Langstroth hive. Follow the local recommendations for Langstroth, and then come here to ask us any questions when it is time to put the super on the hive.
You might also consider trying out this group: