@kitz I did some experiments a while ago on many of the ookla servers dotted around England. I didn't make the mistake of assuming that a North of England server would be the fastest.
For me the London ones, although they are, as you rightly say, are expected to be the fastest, came out with slightly lower readings than ones around the M4 corridor to the west of London. Iirc Newbury or Maidenhead was the fastest. Now this could I suppose possibly have changed if I did my tests before AA did expensive network upgrades in Docklands at the start of the year.
However, I told myself at the time that perhaps the London ones were simply very busy, very popular, either heavily loaded themselves or on pipes that carried a lot of traffic related or unrelated.