Q: I’ve got some sections of tree trunk that I want to carve into figures. Can I carve it when it is still fresh, or does it need to season first? If I leave it outside (in Britain) will I need to use some kind of preservative?
A: Green wood carves more readily than seasoned wood. However, checking that will have manifested itself in the seasoned wood will not yet be evident in the green wood. This can be a source of disappointment as your finished sculpture cracks up; or you can decide you like it. Figures carved from a whole log will show checking in a radial pattern (seen end-on) with cracks starting from the center and progressing to the outside.
Normally, wood intended for uses other than burning is split lengthwise into quarters to counteract this tendency, and the ends are waxed to inhibit dehydration. It is then dried slowly on a rack. If you find cracks difficult to like, then drying the wood first, sawing and planing flat, and joining the flat surfaces with glue is the traditional way to minimize checking.
I would think the climate of the UK would cause your sculptures to commence rotting fairly quickly, especially if they are in contact with the earth. Preservatives are an option, but they tend to be extremely toxic. Sealing the wood with outdoor-quality varnish, paint or resin will help, as will a concrete foundation.