Likely the FTTP/OD will change to 50meg up once G.fast goes commercial.
I guess it depends whether BT's choice of upstream for FTTP using GPON was related to how much congestion they were willing to accept over the PON.
G.Fast's upstream is not shared in the same way.
Or G.fast will be offered at a 30meg upload cap
If there was a risk of that, I don't think they'd have come this far with 50Mbps speeds.