It would be very wrong for the airfield of departure (or whoever you used to file the plan) not to address an IFR plan to the two IFPS (Eurocontrol) addresses.
You can even (very rarely) find that although the majority of the flight is outside controlled airspace, a slot time will be issued!
Why a slot time? Well the bit of controlled airsapace that you plan to cross could have flow restrictions in force.
I do not believe that Southampton or Bournemouth can refuse you an IFR routing through the airspace - there is no minimum overflight level published for the solent CTA as far as I remember. If the airspace will be too busy then flow control measures will be in force and you will get a slot which you have to comply with. Otherwise, a short delay or a vector will get you across the airspace.
Regards,
DFC