Would never and have never done it.
But I would say a good example where it was allowed under old regs, many 70's houses around here have it to outbuildings,
and as far as I understand, if done with storm loops and earthed catenery, a householder could legitimately replace, like for like,
without having to notify and without breaking any rules.
But, my point is, there is NO regulation forcing such a building to be upgraded.
http://www.electricalsafetyfirst.org.uk/mediafile/100423440/Best-Practice-Guide-1-Issue-3-.pdf(point 1.3)