Do you mean you pay separate bills to different providers for "line rental" and "broadband"?
Your broadband provider will contact you some time closer to Dec 2025 to say that they will need to cease your service, unless you migrate it to a new offering. That's because the broadband is linked to the underlying analogue phone service, and that service will be ceased.
Since analogue phone lines are no longer available for sale (since 5 Sep 2023), it's not possible to migrate the existing phone service to anyone else. So you will have to work with your ISP to choose the best way to migrate you to SOGEA, which is broadband without analogue voice. The chances are that they will be able to do it in a way that doesn't result in you losing service for an extended period of time.
The best time to do this will be at contract renewal time.
Maybe I should just get Fibre to the Premies now - and then they don't matter.
(I was going to before but something came up)
If that option is available to you, then yes, that's by far the safest and easiest solution, and future-proofs your connection. You can even run the fibre and copper in parallel for a while, for maximum certainty.
Note that if you have *Openreach* FTTP available to you, *and* your exchange is in a "copper stop sell" area, then you won't be able to migrate the copper service to SOGEA anyway; the only option available to you will be to take FTTP.
If you're talking about FTTP being available from an alternative network provider (Altnet) then think carefully about whether they provide a reliable, good quality service, before choosing them. Some small altnets don't have a clue about how to run an ISP. Whilst the fibre itself may be reliable, the rest of their network may not be. Do your research.
Maybe I should just let them install the fibre to my premisses (if thats what they do?), then get BB elsewhere, saving an installation fee?
You generally won't have to pay an installation fee for FTTP: although some ISPs charge a small setup fee this would apply whether or not you already had an FTTP line. Just order it. The ISP swallows the Openreach installation charge; it's one reason why you're likely to be locked into a 12-24 month contract. (Some providers have 1-month rolling contracts, but then you do pay a hefty setup fee)
1. What actually happens when they come to remove the copper / terminate? my BT LIne?
Your copper line won't be removed. The dialtone on the line will stop in Dec 2025, and the ceasing of that service will in turn trigger the ceasing of the attached broadband. But you can always re-order broadband, from the same or different ISP (unless you're in a copper stop-sell area and have Openreach FTTP available to order)
2. Will my broadband still work from the other company (ie they leave the copper from the cabinet to my premises)
The copper remains. Always. But as explained above, the broadband will be terminated unless you've migrated it to a SOGEA-based service before then.
3. Will I have to get broadband with BT?
4. Will I have to cancel and restart a "new" FTTC / FTTP with my broandband supplier?
Moving your broadband to BT is one option available to you, and arguably might give you a slightly lower risk of service interruption - at the cost of being locked into a 24 month contract with BT. But to be honest, any decent ISP should be able to manage this takeover for you; that is, they will replace the two separate services with one new service. This could be your existing ISP, or a new one.
Ceasing the line completely before taking the new service is very drastic, not necessary, and guarantees you an outage which could be a couple of weeks.