There is no further detail, and for most mixed domestic/WfH scenarios that usage is dominated by software downloads/updates and video streaming or "on demand". If this is purely about work use, then you'd presumably have an idea of download/upload use from the file sizes you use, but things like Teams are a bit more troublesome. If you work on 225 MB per hour, and estimate how many hours of such use, that will give you some idea (or 500 MB if your video conferencing is in higher quality settings).
Mobile hot spots or mobile routers are an acceptable technique for a day or so here and there (I've done this in a VM outage situation) and if only for work use don't normally use excessive amounts of data, but they're still a bit of a kludge. Before spending money on a SIM-capable router, try it out for a day using a phone in mobile hotspot mode, see if the signal quality and speed limits work for you, as well as seeing what that does for data use on the mobile phone. In terms of a PAYG SIM, bear in mind that you need to check the networks terms carefully to avoid losing the pre-loaded credit every month or two, or the SIM being cancelled for non-use. You'd also need to try and stop (for example) Windows treating a hot spot connection as an unmetered one - a computer, game or program update running in the background could chew through tens of GB of data without you even knowing.
The other option is somewhat more costly, but more dependable and won't fall foul of data volume problems, is to run in a 35 Mbps Openreach line, for around £30 a month with a 12 month contract from a competent ISP. £350 a year sounds like a lot, put like £1 a day for peace of mind it may be more acceptable. How much do you value having a backup option?
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