I'd guess that Lem used lieutenant rank from citizens' militia (which was back then the police force in Polish People's Republic), not knowing about rank changes in the UK - it was replaced by chief inspector as you linked. goodreads.com uses "Lt. Gregory" in book summary so it's not a translation issue.

https://en.wikipedia.org/wiki/Chief_inspector#United_Kingdom

https://en.wikipedia.org/wiki/Lieutenant#United_Kingdom_and_...

But the rank doesn't appear to have ever been used in Scotland yard or the MET, which might have lead to why it was used incorrectly.