Bog Standard (2005)

This link has one inconclusive bit of evidence that it might be derived from "box-standard": https://www.worldwidewords.org/qa-bog1.html

  In February 1983, [Sir Clive Sinclair] said in
  an interview with the magazine Computerworld:
  “Luckily, we cannot foresee the day when a
  computer becomes just a standard box. There
  will be box-standard machines along the road,
  but we do not simply have to make those”.

I'd not be surprised if it comes from the existance of '[Ideal] Standard' (the european branch of 'American Standard') brand toilets, someone seeing them and quipping about how it's a 'standard bog' and then 'bog standard, bog' is plausible, IMO).

Between 1929 and 1967, both American Standard and Ideal Standard often omitted the first word on their porcelain products, and just had 'Standard'.

The only real downside I see with this theory is that [Ideal] Standard were far less popular than Armitage Shanks in the UK, although American Standard purchased Armitage Shanks a while back.

This may be a red herring, but Bog Manufacturing of Chicago made automotive tools in the 1920s-30s. (http://alloy-artifacts.org/bog-mfg-chicago.html)

I played in a 6-a-side footy(soccer) team in the late 90s, there was a Belgium team called Standard Liege so our team name was Bog Standard Liege so it was in common usage back then but have no idea where it came from...