Pethidine has been shown to be less effective in severe pain and to cause more nausea and vomiting, the latter because of what it metabolises to. Diamorphine is a much 'cleaner' (fewer side effects) drug, so that's what I wanted to have if I needed it.
There was no way, however, that I was going to have diamorphine at home. If I needed opioid analgesia, I was going to be transferred to hospital as I have seen (and delivered actually, now that I remember) babies born not long after the mother was given one of those drugs. They are fine if delivery is not expected in the next roughly 4 hours as it gives time for mother and baby to metabolise it, but if a baby is delivered soon after administration, it can often be sleepy and not breathe well. That's not a reason not to have a very good drug for pain relief and it is easily managed in hospital with a few extra hands around, but it wasn't something I wanted to risk at home with only two midwives.
I do like your birth plan, Fluffy, Short and to the point! I never believed that the TENS machine would actually work, but it was AMAZING and kept me going from 2am to 5pm and to 7cm, before getting in the pool.