Far too early to tell. A lot of city companies are planning on WFH for months, but still anticipating an eventual return of some degree. They will need to sort out how you use lifts first.....
I'm aware of one finance company (one many would have heard of but not one of the bigger ones) which has allowed a return to the office for those who wish to (especially those who live on their own, have inadequate space at home to work, etc.) however are still talking of WFH being encouraged until at least the end of this year.
However WFH, in the medium term, could actually mean more rail travel. As whilst, say 40% of people could work from home (and no that doesn't result in a 40% drop in passenger numbers as they're likely to still be needed to go in from time to time, with twice a month still being 10% of their working time in an office) many of those would already be driving to work rather than using rail.
If you reduce the number of days in the office you reduce the miles traveled by car and so the cost per mile increases significantly (purchase cost, insurance, VED, residents permits, etc. all have no/limited reductions if you reduce travel). As such the value of owning a car (and certainly a second car) starts to fall significantly.
A small shift in the number of miles traveled by road to rail can, because of the way we travel, have a big impact on the number of miles traveled by rail. As an example a 6.25% shift from road to rail would see Rail's milage increase by 50% of the 2018/19 figures.
Even with a 40% fall that's still going to mean more miles by rail than before.
Now whilst that's likely to result in more longer distance travel and less London and the South East travel (although some of the latter would be offset by the longer distance travel within its boundaries) I'd argue that London and the South East were in need of significant upgrades and so whilst trains will be emptier they will still be fairly busy and fairly busy all day.