AlanM
What's a few extra £ for an IFR departure, guaranteed radar service, a radar handover, and peace of mind.
How about because it's not guaranteed?
Let's move the scenario away from Biggin Hill. How about (as we need a controlled airport for the question to arise) Plymouth to Prestwick - on a weekend.
Get airborne from Plymouth. Non-radar airfield, no nearby radar unit. The weather scenario involved means an IFR departure would not benefit the aircraft at all (
REMEMBER - this was the original question - NOT whether it was legal). Heading north. The next available ATC unit (I'm not including London info here because they can't separate, therefore they don't come into the IFR v VFR equation) would be Ronaldsway (non-NATS) and the next after that Prestwick (non-NATS).
DFC
Having decided to pay Nav charges why not make the best use of the expense and make use of the services provided in return for payment of those charges - London FIS, Scottish FIS, Volmet, VOR/DME navaids are available at all times.
And the above route, with the levels involved, rather negates everything you've suggested.
So - just because we put 'I' instead of 'V' on the flight plan we've paid £80 to NATS via Eurocontrol - and what have we got for it?
This was the whole point of Timothy's original question for those that haven't quite grasped it yet!