Puntosaurus
Like ShyTorque, I too am amazed you have trouble getting a radar service unless you've filed an IFR flight plan - I've rarely been refused without obvious reason when requesting. You must indeeed be unlucky. Whether or not one can get a radar service outside CAS depends very much on how busy ATC is, and I think most pilots use a bit of common sense before asking. If it's a gin clear night, I'm over Aylesbury northbound and Luton is very busy I wouldn't bother asking, though some do - often with the predictable reply! On a !!!!ty night IMC, different deal.
I'm sure I'm not alone in once having forgotten to close a flight plan when landing at a private site until, to my dismay, about 6 hours later. No-one had started looking for me - or indeed called my mobile number, which I always put on flight plans. No drama when I apologetically closed late. So doesn't seem filing even an IFR flight plan affords any kind of close monitoring! Better to rely on talking to any service that's awake, with London information always there and requesting position reports. In fact I'd be interested to know how soon they do exactly what, if you don't report - any offers?
I reckon letting a responsible person know your eta, route and who to call if you've not arrived within 30 mins is the safest option.