So when we search we will never see the split of connections, Seighford exchange will remain as Market A so how do providers know that data is covered by a Market B exchange ?
Do you care in a technical sense? Or a financial one?
You already have the technical answer: because the detail of the Openreach FTTx head-end handover point will be passed on to the CP (BT Wholesale, Sky or TT for example), who will then be able to arrange for the backhaul connection. This happens today all over the country - and isn't just a feature of market A exchanges. There are plenty of "child" exchanges without their own handover point in market B too.
But because you mention markets A and B, I suspect you really care about the finances, and whether you will find, amongst ISPs that distinguish, lower cost packages available to you.
The answer there is "it depends".
Your line will still be classified as market A, as Ofcom decided that, while a large proportion of market A would be covered by fibre by the end of their "review period", they couldn't tell what the effect would be on competition. Their conclusion was to sit tight, for now.
However, Ofcom have set charge controls on BT in market A, that are aimed at reducing wholesale prices each year. The impact of those wholesale reductions on retail prices is unknown.
The ISPs that choose to sell services at a higher price for market A are doing this of their own volition - presumably as a way to cut the cost of their market B service further. Because it is their own choice, they are free to alter the rules they use at any time ... and could change their rules to charge a unified price for fibre-based packages if they wished.