Lets do a Fermi estimate on this to determine the number of GGL (https://en.wikipedia.org/wiki/Fermi_problem). Some data is estimated and some values will be high, others low, but the overall errors should roughly cancel out to give an answer accurate to at least an order of magnitude, but probably 3X either side of the true number.

Data and estimates:

-Others here have reported that spend under the current system spend to attain GGL level is probably in the order of £35,000.
-Gross income to achieve that level of travel spend is in the region of £150,000.
- From UK government data, there are 33,000,000 taxpayers for the year 2021/22. (last year taxpayer and income data is available)
-Of those taxpayers, those whose gross income is £150,000 or more puts them in the 98.5 percentile. This represents 33,000,000 x 0.015 = 495,000 people.
- Of those 495,000 people how many have the travel spend level intensity? Maybe 5%? I chose that as GGL represents a lot of travel, and most people don't do that much travel. They may also chose to travel to resorts or do cruises etc. 495,000 x 0.05= 24750.
- How many spend that much on air travel, as opposed to resorts etc? Maybe 35%? 24750 x 0.35 = 8,663

Thus 8,663 represents the potential pool of UK residents who could become GGL.
-lets say half travel BA, so potential GGL are 4,331.
- I'm going to guess that 2/3 of GGL are UK residents and the rest are ROW for no good reason other that it seems about right. 4331/0.66 = 6562

So a potential number of GGL members is 6,552. If we further assume the true number lies within a 3X range, the potential number is between 2,124-19,656.

I have made a further assumption that of those who travel for business enough to attain enough TP for GGL, that they are at a senior enough level that they could self fund this level of expenditure if they chose to. A junior person doing short haul Y across Europe each week isn't going to make GGL, nor probably earn enough to be able to afford the flying intensity to get GGL. This is a way to simplify accounting for business bought seats vs personal spend and avoid double counting the pool of potential customers.

There are a number of guesses here, I welcome revisions if someone has better data to make the estimate more accurate. I'm also not looking to capture edge cases, but rather where the bulk of the GGL members would lie.

The reported number of 3000 GGL does fall within the range of my estimate, so it might be accurate.

Pushing the spend requirement to £65,000 for GGL will probably push it to less than 1% of the taxpayers could afford it. I think that would drop the potential pool of GGL to a few hundred, maybe 200 people or less.