I would preserve the wood using linseed oil. Not only does it have good preservative and waterproofing qualities when applied to wood, it is completely non-toxic, so even if you get some run-off, you can be confident it will not harm your plants or your health. If you are committed to true organic practices, make sure you source your oil from an organic supplier. Heat-treated wood is an alternative- to find a supplier, google thermowood suppliers. I would strongly recommend against using softwood untreated, particularly in wet climates like the UK, unless you can afford to replace the wood every couple of years.
