Traffic departing from airfields in the Iberian Peninsular area with destinations in the UK are affected by routing requirements in Spain and France, as well as in the UK.
Because of the amount of air traffic generated, especially during the summer period, the Spanish, French and UK Authorities have created air traffic regulations which try to "balance" the loading of Sectors within those airspaces. Depending on the ADEP and ADES (Airfield of DEParture and Airfield of DEStination), aircraft operators 'AO's)will be required to file (with a certain amount of choice) along certain routes. From the Balearics, for example, aircraft with ADES UK North would have the choice of filing a route via Marseille and up through Central France, through "Dead Centre", via Bordeaux, or via a route further west, entering the UK south of Berry Head.
The route selected by the AO would be dependant on several factors - track distance, hence fuel/time constraints; delays, due to Sector congestion; weather.
Certainly, there are different scenario's for Weekday/Weekend situations. The use, and users, of airspace does vary. Normally the Military operate more during the week. However, the main reason for the difference in Weekday/Weekend operations is that more civil aircraft operate at Weekends..
The "Bible" for route planning is the CFMU Route Availability Document (RAD), available from all good CFMU Websites! Each Annex of the RAD indicates the "Terms and Conditions" under which flights may be filed along the various ATS routes. In the context of this posting, LE, LF and EG Annexes (along with an up-to-date map showing Upper ATS Routes) would be good places to peruse.
Hope this answers the question.
MACR CFMU/FP2/Sup.