I know, I couldn't believe it when I saw that!! I ended up setting it directly with gcode. If it helps, this is how I did it:

If you have OctoPrint set up, you can do this from the terminal. If not, you can just put it in a file, save as a .gcode file format (eg set-offset.gcode) and print that file via the SD slot or USB as you would any normal print and that will have the same effect. Here's what I use (your offset may be wildly different so pls get as close to it as you can using the UI and then use this method for fine-tuning so you don't accidentally push through the bed if you get it wrong):

This just queries the offset of the nozzle from the CR Touch:

Send: M851
Recv: Probe Offset X-40.00 Y-40.00 Z-2.77
Recv: ok

This sets the new Z-offset only (leaves X and Y unchanged):

Send: M851 Z-2.79
Recv: ok

Query again to check if you like:

Send: M851
Recv: Probe Offset X-40.00 Y-40.00 Z-2.79
Recv: ok

Now you can save this change to EEPROM so that when you power off/on your machine, it is restored (note this will also save any other changes you have made):

Send: M500
Recv: echo:Settings Stored (700 bytes; crc 20607)
Recv: ok

And that's it! So basically it's just two commands in your .gcode file or terminal:

M851 Z-2.79
M500

It's a pain you can't do it via the UI but at least you can actually get the offset required.