For the benefit of any UK citizens (non US permanent residents) applying:
ACRO certificate application mailed: Friday 16th August
Application receipt acknowledged: Monday 19th August
Certificate received: Wednesday 21st August (I had chosen the priority option)
Global entry application submitted: Wednesday 21st August
Conditional approval received: Monday 26th August
I've just scheduled an interview for a couple of weeks's time, which is when I'm next in the US. Very happy with how smoothly and swiftly everything's gone so far.