You use the FIR identifier, and there is a specific format. If you intend to enter the Paris FIR 1 hour and 10 minutes after departure, write EET/LFFF0110. If you intend to enter the London FIR 15 minutes after departure on the return write EET/EGTT0015.
This is entered into "other information" (item 18). What I was referring to is this (just found CAP 694).
Additionally, for flights to/from France, the French Authorities require the frontier crossing point (the UK/France FIR boundary position) to be included in Item 15 (Route) of the FPL. To assist pilots, the UK now includes the ATS route reporting points on the Southern England and Wales 1:500 000 chart. These can be used as a frontier crossing point. A position may also be shown as LAT/LONG, or as a bearing and distance from a route reporting point or navigation aid.