BMI701EGCC
You will find that ATC fill in the blanks.
How does a pilot know for sure which runway will be used for departure, and therefore which SID....several hours beforehand?
ATC will issue the appropriate clearance.
Likewise for arrival....the exact detail of the arrival will be specified by ATC at the time. Let me give an example; inbound Heathrow from STU direction, normal arrival or STAR is OCKHAM1F......however if OCK VOR is U/S it would be TOMMO1F......in the event that OCK stack was full a stack-swap STAR of Bovingdon1D would be given.
The STAR is worth a read because it incorporates a lot of fine detail such as descent planning guidance as well as speed limit points.
Haven't read your profile, apologies if over-simplistic.
A feature of the London Area Control Centre computer is that it doesn't matter too much what is on the flight plan because the computer will print out or display to the controller whatever route has been allocated as a preferencial route for certain portions of the flight in and around complex terminal areas.
Let me give an example.....inbound to EGGW[Luton] from STU direction.....if you are cruising at or above FL180 you will be given a LOREL1S arrival........below that you will be given a LOREL2L arrival [which is very different ], however the computer recognises the different cruising level and displays the correct level and frequency assignment for the relevant next sector.
The clue is in the title "Flight Plan"...it is a plan not a contract, although it may have heightened significance in the event of radio failure.
Regards.