In the UK most UK wholesale providers appear to be providing a single service over an ONT with Router/Switch functionality provided by an ISP Provided Router.
A multiport ONT is only useful if these network providers intend to provide multiple services over it. For the consumer market its unlikely most users will exhaust their Gig products and using the same network/ONT for more than one service would provide little resilience and no balancing.
A multiport ONT may be useful in MDUs but it appears that the main UK providers are keeping it simple with a single port ONT for each customer.
GPON is a broadcast system and therefore the collective ONTs have to synchronise with the OLT. For security rogue ONTs need to be prevented so hence the registration process which is much simpler to support if controlled internally (serial number/technology/version/config). Even if they had a registration process it would be a nightmare to support devices of unknown origin.
Going forward there will be different generations of equipment in use and possibly multiple versions of GPON using the same fibre network. As a customer you are unlikely to know what is at the other end and it may change.
The only time a multiport ONT may be useful is if the network provider decides to utilise an ONT device as a combined ONT and Router but in that case that would be providing multiple LAN ports not WAN ports (Hyperoptic?).
For manufacture the ONT may be configurable. However for size, cost and their support, single port ONTs are more common.