That's RFC 3021 - it's a way of addressing point-to-point links without wasting a ton of IPv4 addresses. It makes sense as Hyperoptic are relatively new and won't have the same sort of address space allocated as the bigger players.
For what it's worth, you can allocate a /30 mask on a link that is using /31 at the other end, you just won't be able to communicate with what used to be the network and broadcast IP addresses and now belong to another Hyperoptic customer - which is unlikely to cause a problem for a home user connection.