I live in Tamworth (West Midlands) don’t listen to all the crap about modifications and you can’t do this or that with a flow hive it’s all bollocks.
Standard Flow Hive is a Langstroth which is the most used bee hive in the world. UK national hives are different but you’ll get bees and equipment for your Flow Hive anywhere online.
My advice to you is to start studying as you seem pretty blind and keeping bees isn’t just a try and see how it goes you have a duty of care to the bees and to avoid the spread of disease. YouTube is fantastic as is Flows Facebook and YouTube channels. The beekeepers.org is another invaluable source and free for a month just now.
It’s easy to get nucs/colonies on langstroth frames/hives in the UK.
However the current Covid-19 situation will probably set you back a season unless you have a local club/association that can help you out or trap a swarm.
Happy Valley Honey https://www.happyvalleyhoney.co.uk/ is a local bee breeder and honey farm near you.
Owners name is Paul top bloke and has nucs and full colonies on langstroth he’s one of the UKs biggest suppliers in langstroth. Website is not advertising sales of bees for now due to the requirement for social distancing.
You don’t sound ready for bees yet and dont take that as an insult but you need to learn the foundations and not knowing about hive sizes but buying a FlowHive etc shows you need to study a bit more.
Won’t take long but if you want to enjoy keeping bees it’s better to have the basic understanding otherwise you’ll end up with a very expensive empty wooden box.
Message me anytime for help I’m no expert but nothing that can’t be worked out with help