Hi Martin,
I don't garden for a living but have a very large 'garden' in rural France, it is almost a full time job looking after it.
I find that working all day in the garden and particularly in heat or wind and rain reduces my need for insulin considerably. You may need to reduce both your basal and your mealtime insulin.
Obviously some activities are less likely to cause drops but even things like standing in a potting shed pricking out seedlings seems to take more energy than you would think!
Often, I find it necessary to eat something like a piece of fruit after a couple of hours to keep my levels up. (well I did before I got a pump, now I can use a temporary basal...a pump helps enormously. )Test frequently and keep hypo supplies to hand. Also make sure you are well hydrated (though I assume in the UK you will be plied with cups of tea!)
Be particularly careful to check glucose levels before doing anything remotely dangerous like using a hedgecutter, a chainsaw or climbing trees!