Merino all the way. Go for a heavier GSM weight and long sleeved, they almost self regulate your temperature so you shouldn't overheat or get too cold, they also don't smell like artificial fibre. Keep an eye out for Planet X doing a special Merino event like last year. Very good value.
Decathlon were doing some at the weekend for £20 reduced but tbh I would get a slightly heavier weight than theirs.