Hello Diana, and welcome to the Flow forum! You did the right thing to come here.
Don’t fret, everything will be just fine! You could order the UK National unit, and I actually love British National or WBC hives, because they are so much lighter than Langstroths (which is the Flow hive size). Having said that, you can easily adapt National frames to a Langstroth hive, using this clever trick that a very helpful forumite posted:
So if you can only buy a nucleus on National frames, you can adapt them to fit your Flow classic. Having said that, there are UK beekeepers who sell nuclei on Langstroth frames, so you may not need to adapt them. You have some time before next Spring (the best time to install bees in a new hive) to join a local bee club and ask around. I would advise you not to mention the Flow hive though - many traditional beekeepers are very suspicious and dismissive of Flow hives. Just say that you have a Langstroth hive, and if anyone else does too, you would love to buy a nucleus from them. 
Keep asking questions, and we will help as much as possible. There are several other UK beekeepers here who may also have some advice for you, including @HappyHibee and @Paras.