Going from Fairoaks to Bournemouth why choose a route GWC, SAM, BIA which is mostly outside controlled airspace and requires a route through the Southampton traffic.
Why not route west initially talking to Farnborough and then route south towards bournemouth who can vector you to the ILS without ever getting in the way of solent.
If you do not have BRNAV then you can make use of OCK and CPT vors to get you in range of the BIA.
You have obvously recognied the problem and that is why you asked the question. For flights such as these (mostly outside controlled airspace), ask yourself would you route that way on a VFR flight - unlikely. Look at which way your would route VFR and then tweek it if required to get you on some VOR radials.
Even on a weekday, this route is better since you can get a service from Farnborough, Boscombe and Bournemouth.
For the flight plan - yes file one. Your IFR flight will be entering controlled airspace and the preferred method is for the flight plan to be filed before departure. One could argue that it is mandatory but that is a separate issue. Speaking generally, unless the IFR flight is 100% outside controlled airspace then always file a flight plan because you may find that the bit of controlled airspace you want to fly through on the day has a flow regulation and you are thus required to have a CTOT to depart.
When you receive your ACK message for the flight plan, make a note of the unique ID number assigned to the flight plan. Then if someone claims that they have not received a flight plan when they should you have the confirmation and can give them the ID number so that they can retreive the data from the system.
Don't forget that PPR and airport slots are a totally separate issue.
Regards,
DFC