Level Attitude says 'Holding another Instrument Qualification, per se, gives no reduction to the CB IR requirement'.
This is not true.
The EIR training can be used as credit against the CB-IR 40 hour (s/e) or 45 hour (m/e) course, as can the IMC rating, or any other ICAO IR. Any PIC IFR hours flown using the EIR, IMC or ICAO IR can also be used as credit against the CB-IR course.
All of this can reduce the CB-IR to a minimum of 10 hours.
You are right that the ATO is required to undertake a pre-course assessment but the reductions detailed above in course hours should be allowed. The pre-course assessment will give the ATO some idea of whether someone is likely to achieve standard in 10 hours, or whether they are likely to take nearer the full course hours, but not exactly how many hours they will require.
Whopity notes that the EIR skill test will cost £775. What a cheek, the same price for a test which takes half the time and is nowhere near as complex. That's the CAA for you!