Responding to the first post only:
Is there actually any difference between filing IFR or VFR when outside controlled airspace and if not is there any point in filing at all (above the advantages of filing a normal VFR flight plan)?
It's a very good question, and firstly it depends on why/whether you actually file a flight plan.
An IFR flight plan should be sent to Eurocontrol, and is distributed to the IFR sector controllers enroute e.g. London Control. But if you file it for some low level which is "obviously" OCAS then you are not going to get a service from LC. I don't know what exactly happens here but I believe the way it breaks is this:
When you file a FP to Eurocontrol you get an airways squawk allocated. When you get airborne with this squawk, and it is picked up by one of the radars, the IFR FP pops up on their screens, and then when you call them up they know about you and send you off into CAS, to your filed level/route etc. This is what happens when you fly from say Bournemouth to Berlin at FL150. Very smooth and simple.
But if you never go up with that squawk (because you filed an IFR FP but you never picked up the airways departure clearance) that squawk will never be seen, and after 30 mins past EOBT the FP gets dumped.
If you depart on an IFR FP from an airport
capable of airways departures e.g. Bournemouth and the FP says 2000ft then Bournemouth tower will look at this FP and will realise you are just an amateur playing low level (probably an IMCR training flight where the instructor got you to file an IFR FP for fun) and is not going to pass you the airways squawk because they can "obviously" see that you are not heading for any airspace run by LC. If incidentally you did later try to get that flight elevated to airways it won't work because it got dumped...
So what is the point of filing an IFR FP OCAS?
None really. No enroute unit is going to see it, and because it went to Eurocontrol it is likely (??) that even the destination ARO will not get it (because it gets dumped 30 mins past EOBT if the allocate squawk is never seen).
An airport operating the IFR rules properly (typically, one in Class D) will not let you depart VFR into sub-VFR conditions, so there you have to do an IFR departure, and the existence of an IFR FP will be irrelevant.
It's all a bit of a muddle and this is why people go to the huge hassle of getting the IR. Then one gets the whole-route clearance, everybody knows about you, etc.
The only advantage of filing a FP (VFR or IFR)
OCAS is that somebody can dig it out if you crash somewhere, and go looking for you in hopefully roughly the right area. But, in the UK, somebody still has to raise the alarm - a FP is automatically closed even if you never arrive!!
Therefore, I never file flight plans for flights within the UK - cannot see the point.
Most instrument-capable UK pilots just depart VFR, and in Class G the transition to IFR is purely inside your mind, so no need to tell anybody. Just disappear in some cloud... If you are receiving a service then you need to advise them though, but in Class G ATC has no power to stop you anyway.
Going abroad, one has to file an IFR FP to enter any IMC or to fly IFR, but the IFR rules are different there anyway. You need the full IR, and an Eurocontrol flight plan, for any IFR flight.
C/Tower I have sent you a PM.