I would agree that Netflix is the best of the three, there is so much variety and some great original content (eg The Crown, Stranger Things). The software works really well too. I do find in general it is biased towards American content though. If you say you prefer British drama, do you even need a subscription? Assuming you have a TV licence, there is so much on BBC IPlayer now, not just stuff broadcast the last few weeks. Have you explored it?
I would say Britbox for British content, the trouble is that over 95% of their content is old stuff shown on UK TV over the last few years, so you may find you've seen most of it. But you could have a free trial and see what you think. Also if you are on EE, they were offering 6 months of Britbox for free
Now TV is basically Sky programming so their big shows tend to be American. Some good stuff though, currently watching The Undoing with Nicole Kidman and Hugh Grant. It's a fraction of the price of actually subscribing to Sky, and it used to be that if you tried to leave you would always get a ridiculously cheap offer to stay. Not sure if that's still happening, but you aren't tied in anyway. Try Googling for Now TV offers. My main complaint about Now TV is that the software is a bit clunky, eg doesn't have the smooth 'rewind 60 seconds' function that both Netflix and BBC IPlayer have.
Amazon Prime is not bad, there is plenty of good content especially films. There is also a lot of absolute dross for some reason. Don't like how they push you towards paid-for content on top of your subscription, as pps said.
Do you have family or friends in different households that you could share a subscription with? It's a bit of a grey area but I do this, DD and her partner have Now TV and we have Netflix and we share logins. Means that we get to try more things out. I wouldn't share accounts with more than one or two households though.