What I took....
Went last June. Rained all the time, honest

picos were flooded and the Pyrennese were draped in pea soup fog.
Still had a great time though
Legal stuff..
Driving licence, both parts (and take a photo copy as well).
Insurance, mot, log book V5 with photo copies.
Passport!
I took a hiviz stuffed in the panniers, never got asked for it though. You are supposed to have bulbs and tools to change them. Took em, never got asked.
Sensible stuff...
Phone and charger, credit card, breakdown cover.
Tyre plugger and a pump just in case. Waiting for hours for a breakdown truck or 5 mins with a plugger kit.... You choose. (stop n go do a good small kit).
Security stuff.
Waterproofs......it rains over there as well. Spare gloves!
Done many 1000s of miles over there, feel free to ask.
Enjoy..........
Roger.