CM, you're quite right, thanks for correcting me.
Knobby, I think the reason that I don't tend to use 1013 very often is at least partly because the transition altitude (see, I got it right this time!) is 6000' around London, and I don't often get that high! Certainly if you're spending lots of time above the transition altitude, especially in level flight, there's no reason not to set 1013. But it's not essential.
Also, CM has a good point about the base of controlled airspace.
FFF
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