I will say that the amount they suggest was spent since 2017 is not really so much in terms of all the police forces in the nation.
I agree. The real issue is the workload it causes to individual police forces, and, as you say, what they get in return - other than misleading information about the Equality Act. And even if the information given by Stonewall was accurate, are they the best source for that kind of advice, when their sole focus is LGBT rights?
For comparison, how much does the police spend on disability consultancy services?