Civil servants who are providing face to face services to citizens - border control for example - are all in their respective workplaces 100% already.
Civil servants who have to physically be in a particular building to be able to carry out their role are also in that building every day. For example, during lockdown there were high Covid levels in DVLA because of how many of them had to still keep coming into the building.
Staff on helplines are going to be amongst the lowest paid, so staff turnover is likely to be an issue there, similar to private sector helplines. Getting through to my credit card provider for example takes between 45 minutes to 90 minutes.
The civil servants who are working from home are those whose role involves using a laptop all day. Same as in many other private sector roles. They don’t need to be in a particular office building to use a laptop, produce reports, send emails, do Teams meetings etc.
There already weren’t enough desks for everyone pre-pandemic due to cost cutting, the difference being that you would leave your workstation and go to a meeting room, which meant staff could desk share whilst colleagues were off in a meeting. They took out even more desks once homeworking was brought in. And desk sharing when more staff are in the building than there are desks is more difficult now as meetings are usually all at your workstation via your laptop.
The increase in staff numbers was due to Brexit, because of the various functions transferring back to the UK from the EU. It just happened that the pandemic came along hot on its heels. So they talk about pre-pandemic staff numbers rather than pre-Brexit. We haven’t seen exponential growth since the pandemic by the way - civil service numbers are not 16 times what they were in 2020.