This page contains resources that I use for flight planning purposes. It is primarily put here for my own reference although I would be delighted if you find these helpful.
AIP and other administrative information
UK AIP: https://nats-uk.ead-it.com/cms-nats/opencms/en/Publications/AIP/
Spain AIP: https://aip.enaire.es/AIP/AIP-en.html
Portugal AIP: https://ais.nav.pt/wp-content/uploads/AIS_Files/eAIP_Current/eAIP_Online/eAIP/html/index.html
Portugal VFR Manual
EuroControl Network Operations Portal
Air Navigation Inter-Site Acronym List
Navigation / PBN
AUGUR GNSS Performances Prediction Tool: https://augur.eurocontrol.int/status/
Weather
Met Office Aviation Briefing Service
RASP (gliding): Regional Atmospheric Soaring Prediction
Windy
ECMWF Charts: https://charts.ecmwf.int/
NOTAM
UK NATS Notam: https://nats-uk.ead-it.com/cms-nats/opencms/en/NOTAM/
NOTAM info map: https://notaminfo.com/ukmap
Flight Plan
UK NATS AFPEx Login: https://www.flightplanningonline.co.uk/
Spain Icaro login
Blank ICAO flight plan form (PDF)
GAR, Immigration, Customs
UK Submit GAR: https://www.submit-general-aviation-report.service.gov.uk/
ICAO Annex 9 General Declaration PDF DOC
Charts
Spain VFR charts: https://insigniavfr.enaire.es/