You may feel it too expensive, but I got a Fairphone from the Phone Co-op. They also do other phones (cheaper) and will talk you through your choice.
We 'tax ourselves' by buying ethically when we can - that is our choice. The other reason for choosing the Fairphone is that every bit of it is replaceable, so it should last for a long, long time!
Having bought it outright, we then use Phone Co-ops Pay As You Go - they also do a very basic 'bundle' which I buy when we go away. We have no mobile signal at home, so use the phone very little, but it is very useful when we are away from home.
Phone Co-op also do monthly payments if you want to pay less upfront -they are more flexible and less 'pushy' than other providers I find.
And yes, I kept my number - it did take a week or so to 'port' the number across, during which time we had a 'temporary number' on the phone. You have to ask your current provider for a PAC number, but Phone Co-op talked me through it.