It does seem odd that one can't get a licenced person in to do it. As I understand it, without any training, I could legally shoot one of my own pigs, running the risk of injuring/distressing it but someone familiar with the process couldn't.
One probably can ! Interestingly the advice changed in 2011 - although no laws have changed.
The 2011 advice says "26. It is unlawful for a farmer to use the services of an itinerant slaughterman both to slaughter his animal and to dress it. This is because the slaughterman would be supplying goods (i.e. a dressed carcase) in the course of his business."
This in itself is accurate, the key here being that what is presented by the slaughterman differs from what he was handed.
What the 2011 advice doesn't say is what the 2009 advice said in exactly the same paragraph viz:
"24 It is unlawful for a farmer to use the services of an itinerant slaughterman both to slaughter his animal and to dress it. This is because the slaughterman would be supplying goods (i.e. a dressed carcase) in the course of his business. However, if the slaughterman did no more than kill the animal, leaving the farmer to dress and cut the carcase, the Courts might be less likely to conclude that the slaughterman was supplying goods (and more likely to be supplying services). If so, this activity might be held to be lawful, (i.e. the EU Food Hygiene Regulation 853/2004 would not prohibit it) although the issue is far from clear. The EU TSE controls would in any event apply."
As I said earlier no laws changed in between, the wording was conveniently (conventiently for the FSA who don't want this happening) shortened.