So I've been trying to repro this but it never redirected me back to tower this time. Instead, it would repeatedly give me permission to push and start, it never progressed beyond that point. Though one thing did occur to me, in all of this, pressing CTRL+SHIFT+\ didn't seem to do anything despite it being clearly indicated a Roger/Read back command in the manual.
This didn't seem right to me so I thought it best to check key mappings and sure enough, the default CTRL+SHIFT+\ key didn't correlate on my keyboard as it wasn't registering when pressing. As soon as I remapped that to the correct CTRL+SHIFT+\ for my keyboard, I was able to read back and confirm instructions. So all in all, this appears to just be an issue with keyboard mappings.
If I may make a small suggestion to help future new owners, it might be useful to sanity check these important keyboard mappings on first run. From my perspective, I didn't realise just how important they were having never used this before. While they did appear to be right, it seems that the backslash key is in different places on keyboards throughout the world - so technically, this was wrong for my particular keyboard.
Thank you for your help here, I've FINALLY been able to ditch that dreadful default ATC!
Best, Steve