Departing IFR, and filing an IFR flight plan would probably help in getting a RAS, though I am not sure on how the system works with regards to routing IFR flight plans. Probably a LARS unit wouldn't get a copy. Every time I've pre-filed though, it certainly seems easier to get a RAS.
Pure co-incidence. Flight Plan filing has nothing to do with service received. If you say you're IFR you can get a RAS, but only then. Flight Plans (in the UK) only go to the centre's if you've routed airways, airfields of departure and destination, and anyone else that you may have addressed it to - though as most pilots leave the addressing to ATC then it normally only goes to who I've said above.