Because of the vertical profile posted by Dave Reid on Page 2, in particular the 500ft height gain on base leg, combined with a knowledge of the conditions on the day and a look at the topography. It was just an observation, having seen that plot.
For what little it's worth, I flew from Bembridge to Elstree via Farnborough in a C-150 on Friday between 1500Z and 1700Z. I experienced plenty of thermal activity along my route between 2,000 and 3,600 ft amsl, and had no trouble holding altitude to within +- 100 foot.
I'm a PPL student with 15 hours, so if I can do it in my spamcan, I'd be amazed if an experienced pilot in a much heavier, faster jet a/c would have been unable to do so.
But, just an observation from somebody with only a tiny bit of knowledge, so apologies as always if I've missed a salient point here and there
/Alex