My plan is to cease my existing copper line once I have an FTTP connection up and working OK. I had kind of hoped that OR would remove the copper cable at that point, but from what you are saying that's unlikely? It's overhead from a pole.
Typically (but not always) they will replace an existing overhead copper cable with a "hybrid" cable which contains both copper and fibre. If it's underground fed through a duct, you'll have both copper and fibre cables.
Is my assumption that eventually OR will build fibre everywhere correct?
That's the long-term aim, yes. By end of 2026, the expectation is that 80% of properties will have Openreach FTTP. However, getting to 99% or more will take considerably longer.
I imagine there's some Ofcom regulation that means they can't just pull the plug on folks who still have copper without providing some alternative, especially as the cohort is likely to contain older and vulnerable customers?
Removal of copper has a defined process. Roughly speaking, once an exchange area reaches 75% FTTP coverage, Openreach can start a "stop sell" on new copper connections to any property which has FTTP available. About a year later they can stop renewals of copper connections, and this will eventually force people onto FTTP - for those who can get it. But that doesn't mean they'll be ceasing copper services where FTTP is not available. They are likely to be in-filling for a long time.
In the long term, they'll have to do something about the last few remaining copper-only connections. (Note that by the end of 2025, this will be primary GEA FTTC - without voice from the telephone exchange. I am unclear as to when exactly exchange-based ADSL/MPF will be phased out)
In some areas Openreach may swallow the cost of upgrading to FTTP, perhaps with government handouts. In other areas they might move people onto something else like 4G/5G/6G. Over the next few years, the coverage of mobile networks to rural areas is going to improve substantially as the Shared Rural Network is rolled out.