The availability of FTTx and WBC ADSL2+ are not necessarily connected, not least as FTTx is an Openreach product to which BT Wholesale connect, whereas WBC ADSL2+ is an entirely BT Wholesale affair.
Openreach's strategy seems to be to run the fibre network back to neighbouring exchanges where it is sensible to do so. This reduces the complexity of the backhaul network needed for fibre, and if fibre-only areas become possible, will allow disposal of the redundant copper-only exchanges, with the receipts presumably going towards the cost of deploying the fibre infrastructure. I believe BT Wholesale endeavour to have their WBC network ready to connect to new FTTx infrastructure from the first day it is available.
BT Wholesale can only provide WBC ADSL2+ by running the WBC network to the exchange where the lines terminate and install a new MSAN there to provide ADSL2+ service.
If FTTx in your exchange area runs from one of the neighbouring exchanges, that won't bring the WBC network to your exchange, but it may provide much of the routing for new fibres if new fibre is needed to bring WBC to the exchange.
In other words, the fibre deployment may require the 21CN WBC network to be extended to your exchange, it may make any new fibre runs needed to bring that network to the exchange easier to install, or it may not affect the ease of deploying 21CN WBC at your exchange in any way.
Ultimately, BT Wholesale want ubiquitous 21CN WBC ADSL2+ deployment, so they can completely get rid of the legacy 20CN platform. They are already planning to remove 20CN equipment and network links in some exchanges where 21CN WBC ADSL2+ has been available for a while, not least to use the equipment as spares to keep the rest of the 20CN network running until it is replaced.
Meanwhile, it may be worth putting your existing ADSL line statistics into MrSaffron's calculator
to see what increase, if anything, ADSL2+ might offer in your connection speed. This may help in deciding whether to go for FTTx when it's available or to hold out for ADSL2+.