In the UK they can clearly generate an IFR plan if they want to. However, it seems a major PITA for the guys. On an occassion where they can't find the plan (or you show up on a c@cked up IFR handover from some other FIR) the controllers can sort it, but it does seem to cause a lot of faffing in the background.
In the US, the system is pretty simple, a couple of keystokes generates a fragment flightplan, assigns the unique SSR code and you are good to go (i.e. for a clearance to go VFR->IFR to shoot an approach through a cloud deck). This is the same process they use if you call for flightfollowing (which needs a basic FPL inorder to assign the unique SSR code - AIUI). If you decide you want your enroute flight to be IFR, I think it is better practice to call FSS and give them the full plan rather than try and get a 90 second slot of airtime from NY TRACON!