The AFPEX link will take you to the AFPex site, the online flight planning service provided by UK National Air Traffic Services in order to submit your flight plan.
The requirements for flight plans in the UK are set out in the CAA's publication CAP694.
Flight Plans can also be filed by fax, or even in the air, using the information set out in the standard form which predates the online services. Or if you are at an airfield with an ATC function they will usually file the plan for you. This service is provided at Lashenden Headcorn, for example, from where many pilots new to crossing the Channel start their international journey. A copy of the form can be found here.
It is a legal requirement to carry a copy of the CAA interception interception procedures with you when crossing an international border.
Eurocontrol provides links to Aeronautical Information Services for all European countries.
Because the UK is not in in the Shengen Customs union with the remainder of Europe, the customs requirements for travelling between the UK and Europe need to be studied carefully.
Strategies for Crossing the Channel are described here.
NOTAMs can be found on SkyDemon and SkyDemonLight (UK Notams only) and other proprietary platforms.
Other useful services we have found include NotamPlot which is a PC application to download and view UK NOTAMS. NotamInfo is an online service that can email you details of Notams on your route or can graphically show Notams overlaid on a map for many countries including UK Notams and France Notams.
Details on using the radio abroad including the radio calls to make in the circuit.