Last month we did our son's first British passport. He is resident in Thailand and it meant having to do paper form, two actual photos (not digital), countersignature and submission through VFS Global who act at HMPO in Bangkok. The guidance (Thailand is Group 2) asks for a full colour copy of any uncancelled non-British passports so we included a full colour copy, including front and back, of his Thai passport.
Our appointment to submit the application at VFS Global was on Wednesday 14th August. It had arrived at Liverpool HMPO by Friday 16th, on DHL via BKK-BLR-LEJ-EMA. I got an email on 29th August to say the passport was ready to pick up at VFS in Bangkok. I was impressed with that turnaround.