Should I set wooden porch pillars into, or on to concrete footings?
Set a steel pin (1-2cm diameter) or post-base-bracket in the concrete. Drill a hole in the bottom of the post for the pin, or attach the bracket to the post bottom - no lateral movement, also no accelerated rot. Rot is effectively guaranteed if you set the post into concrete. Brackets designed for the job maintain a small space between the concrete and the post. If you don't use that type of bracket, or you use a pin, set a small square of tarpaper/roofing felt/asphalt shingle on top of the concrete, under the post, to reduce water transport into the post from the concrete. A quick search indicated that post-base-brackets seem to be available in the UK, so that's the best option - tie the lower part of the bracket into your reinforcing mesh before pouring concrete. There are several typical options, this is one type (image from Simpson Strong-tie)