Basically the OR website says up to 1.6Gbps but when trying to order via Zen they say no it’s capped at 900Mbps. Just wonder what’s going on here and why the discrepancy? I can order this package from Vodafone and EE website it seems.
Sure.
Some of the OLTs that provide both the backhaul for FTTC cabinets and for FTTP Zen only have a single gigabit connection to for their own network. Where this is the case they'll offer at most 550/75.
They may offer the 900 service using BT Wholesale. They can't offer 1.2 or 1.8 over BT Wholesale as they don't use the BT Wholesale product needed to get it.
Vodafone have their own 10 Gbit connection to the OLT so can order the 1.8 Gbit direct from Openreach.
TL;DR Zen don't have the capacity to sell more than 550/75 on their own connection to Openreach and their wholesaler will only sell them 900 through theirs, so that's all they can do.