Just out of interest, why are Openreach currently building a product as part of the nationwide gigabit rollout, where the end user can only purchase up to a 900mb package?
They may be missing something here, unless 900mb counts as a gigabit connection
As Openreach are a network builder and wholesale network service provider, they sell amongst other things broadband products to their customers, the ISPs/CPs , which you and I as and-users buy services from.
The GEA-FTTP network that Openreach are building nationally currently operates in excess of "Gigabit" in both downstream and upstream directions. Actually 2.488 Gbps in the downstream direction and 1.244 Gbps in the upstream direction.
Openreach publish product guides, called SINs (Suppler Information Notes). For GEA-FTTP the relevant note that describe the FTTP service is SIN 506. You can find it
here.
Openreach have taken the decision to provide FTTP Products at up to a maximum of 1000/220 Mbps. Within the SIN - for the highest tiers 1000/220 and 1000/115 Openreach state as follows:
"The 1000M product provides a max Ethernet frame throughput of 987Mbps (excluding IFG and pre-amble). This is limited by the 1000BaseT interface and Ethernet framing overheads"
Basically as pointed in posts above, these are inherent limitations on the "net" data throughput of an Ethernet interface - which is the interface that is used to connect the customer.
Now Openreach could take the decision (admittedly unlikely) to provision higher download (and indeed upload) data rates on the current network. Other providers like CityFibre already do this effectively offering symmetric upload and download using the same GPON technology as the Openreach network. Indeed they could actually over-provision the download above 1 Gbps (like certain providers outside the UK) - however this would necessitate a different ONT which wouldn't be capped with a 1000BaseT customer connection.
The other matter to reflect on here is that the 'network' really is a passive network from the exchange to the user premises. This will undoubtedly remain for as long as the copper network has. It is entirely possible and probable, that at some point when it becomes commercially sensible that Openreach will "upgrade" this Gigabit-class network to become either a 10 Gbps, 25 Gbps or 50 Gbps (all current GPO=ON standards) network on the backbone - whilst simultaneously maintaining and running the original "Gigabit" connections to existing customers.