the SIM7000 works with 2G and NB-IOT / Cat-M1. The latter two are techniccaly 4G / LTE technologies but they are “special”. When Hologram says LTE they mean LTE Cat-1, 2, 4, etc. They will list Cat-M1 and/or NB-IOT separately (See their coverage in USA).
Right now, Hologram does not Support NB-IOT on any network (due to the volume of questions of this nature on the forum I think they need to do a better job making this clear). It is on their roadmap but it is more up to the carriers to allow Roaming onto their NB-IOT networks which currently no implementation does. The UK has NB-IOT.
Hologram does support Cat-M1 in most areas and even in areas they dont explicitly list. For example Cat-M1 works with Telstra in Australia even though they dont officially list it. Unfortunately there is no Cat-M1 in the UK. See: https://www.gsma.com/iot/deployment-map/ for an up-to-date deployment map NB-IOT/Cat-M1.
Right now your only option is 2G with your modem and Hologram. The only other option is to get a Vodafone simcard that supports NB-IOT or use a different cellular modem that supports “standard” LTE. Note these are considerably more expensive, like an extra $20-50.
Best of luck.