If your sneeky you can get more money knocked off the line rental most months for poor service in certain areas (if you get poor service of course). I joined Three in Nov 2018 black friday deal £20 unlimited everything (£70 cashback if i remember correctly too), however data didn't work at home address and often found calls couldn't be made or received there either. I complained each month and got between £5 - £10 knocked off for the first few months after which i managed to get them to reduce the line rental permanently to £13, but kept complaining resulting in more credit again which resulted in me being in credit for a few months in a row so no line rental for me to pay out of my own pocket.
If you want to try the above, use the live chat feature on the three website, i found you sometimes have to try speaking with more than one person at three to get what you want
I've now left Three and moved over to O2 as they seem to be only network who give a decent service at my home address and i need that with lockdown and working from home. To be fair, anywhere else but at home Three were really good for me and provided a good signal and fast 4G speeds.