Be aware that many YouTube videos are made by beekeepers who aren't based in the UK and may be keeping bees in completely different environments to those in England. Not everything they do or say will apply here. There are also plenty of people perpetuating myths about bee behaviour regardless of the facts. That doesn't mean it's all junk, but don't assume everything you watch is correct or will apply to you.
I'd second the recommendation for the Haynes book. There are one or two things I disagree with, but overall it struck me as a decent introduction to beekeeping.
Two (or even three) colonies is a helpful minimum number to aim for, but the bee police will not turn up and you won't have an accident on the stairs if you only have one. Two or three gives you a few more options if something goes wrong.
I'd strongly recommend doing the beginner course at a local beekeeping association. Not because everything they'll teach you is right, because sometimes it won't be. You will meet plenty of people who you can ask for help you out if you need assistance in a hurry however, and you'll get a chance to find out what it's like to open up a hive full of bees before committing yourself to the outlay of your own hive(s). Some people love the idea of keeping bees right up to the point where they find themselves in the middle of a cloud of them and are never seen again. Not everyone finds the reality matches up to the expectation. If you can get to see what it's like this year as Dani suggests, all the better.
James