Openreach's "where and when" page
is best for details for each exchange. However, those pages haven't been updated since early April (they had been updated every month until then).
But the ability for your line to get FTTC depends on your cabinet too. There is no official list of cabinets (but Openreach's data did escape into the wild in early May - and was a good snapshot of the plans then), so the best checker is the BT Wholesale one. You have one link, but i tend to use this one: BT Wholesale DSL Checker
Those pages tend to hold accurate data - or at least the most accurate data available to the public. But the plans do change often - usually by being delayed.
Elsewhere on the web, the data usually suffers some problems:
I have found that "Marketing" pages tend to hold dates that were valid at the time of publication, but then don't get updated as the plans change - this is true of the BT Retail pages for their Infinity product.
The Infinity product has another problem - for marketing reasons, BT Retail won't sell it to you if the prediction is for less than 15Mbps. And because of those marketing reasons, a prediction of less than 15Mbps is shown as "no availability" on their checker.
However, the checker for BT Wholesale will include predictions down to 5Mbps - as these can still be ordered by other ISPs.
SamKnows doesn't seem to know, in this case. Their FTTC data for exchanges doesn't seem to be based on anything published by either BT Retail, BT Wholesale, or BT Openreach.
I suspect that their pages get updated - at some point - following the BT press releases when exchanges are added to the plan. However, they don't seem to get updated either when the plan changes, or when the exchange starts to go live. And they don't seem to have *any* data that shows the service at any level (area, cabinet, or line) below the exchange level.
So to answer your last questions, on when you get a definitive date, and when you can order:
The BT Wholesale DSL checker is the place to go. Usually, this changes as follows:
1) It starts by giving you no information about fibre
2) It then changes, and starts to give you a rough estimate. These are usually dates at the end of a quarter (31 March, 30 June, 30 September, 31 December).
While the checker shows a date like this, it is still only a very provisional date - even if you get to within a week of that date. Do not treat these as definitive dates at all!
3) If the exchange and/or cabinet RFS (ready-for-service) date gets delayed behind the scenes, so that the provisional date above becomes impossible, the published date will be changed to the next end-of-quarter date (so still be provisional). This tends to happen within the last 2 weeks of the quarter.
4) When the exchange and/or cabinet are close to real RFS, the published date can go through a quick series of wacky changes (moving to 6 or 9 months later, then back again), or even disappear.
5) Finally, when the exchange and/or cabinet have a proper RFS, the published date will change to a real date (ie not an end-of-quarter) - usually sometime in the next 2 weeks.
6) When that date arrives, you will be able to order with any ISP - unless the prediction is for less than 15 Mbps, when you can use any ISP other than BT Retail.