My 80 year old father in law in Belgium is in the process of cancelling his landline, something he's had for the last 60 odd years. He says few people ever call him on it, instead mostly on his mobile and Whatsapp. My argument for not having a landline myself is why should I pay some €20/month for a line so that others can call me cheaper? And if I make any outgoing calls those are over and above the €20. If you want to call me bad enough, you'll find the money to call me on my mobile.
Now when it comes to an area with little to no cell signal but decent cable/fibre internet, I'm sure the UK has VoIP phone providers. In other words, you get a land line number with land line rates, it just works over internet protocol. But again, you will almost always pay a fixed fee just for having it whether you use it or not.
I know in places like South Africa (where I was born), if you have a geographic land line number from Telkom, you can port it over to a VoIP provider. I would like to think you can do the same almost anywhere else. What I also know about South African phone numbers is no one like Telkom actually owns them, they are licensed out to them by The Independent Communications Authority of South Africa. (ICASA) Again, I would like to think you can port numbers in the UK too. So I would check around before you believe someone who says you can't keep your number. I have ported mobile numbers between many networks in both South Africa and Belgium.
And no, I don't think your current provider is deliberately turning down the tap on your current connection. Older twisted pair and coax copper cable systems are just not able to carry the load of internet traffic these days. With fibre, every connection has its own dedicated line direct to the host node, whereas with coax, you share the same line with everyone else in your street or suburb. At least you can't see each others traffic, but think of it like a freeway shared by lots of private cars. Fibre anywhere is almost always unthrottled, unmetered and ununcapped, with a choice of speeds. Here in Belgium we can get 1Gbps uncapped for €45/month. However, we are still with coax for now because it still works for our needs, and I'm still waiting for better fibre deals. But get fibre if you can, you won't be sorry.