I agree with your overall point, OP.
Out of interest, these are the numbers of card holders going through LHR T5 from a single September day in 2015 (i.e. very out of date I'm afraid!):



BA operates around 250 short haul and 80 long haul flights per day from LHR.
I am not sure what the ratio is for T5/T3.
Say a conservative average of 20 SH CE seats and 50 LH CW seats that gives a total of 7,000 passengers eligible to use the Galleries club lounges.
You would then need to know what % of SCHs are already in club (high, I would imagine, maybe 50%+) and then work out the % of economy travelling SCHs in the lounges.

It seems like it might be around 16%?
You would also need to take into account the % of GCHs in club, who presumably would be in the GF lounge, and also consider any otherwise ineligible guests.