A lot of it is in your head. I think at normal pace the wind drag is almost negligible, but it surely contributes to RPE. It helps me to remind myself of this fact during a run or race.
Effect of rain can go either way. On a hot day it obviously is a welcome cooling effect. But since your in the UK it’ll probably be cold rain, which can be terrible - or push you to go hard af just to keep warm. Make sure to keep hands and arms warm, I sometimes loose feel in my upper extremities because they’re exposed to rain and wind and receive relatively little blood during a run.
A final word of my motivation my track coach in high school used to give us on rainy days: How far do you think you’ll come in life if you let a few liters of water hold you back?