I understand why you would do that in class G, so if talking to ATC giving you a traffice service in class G , and you were in cloud, on a UK IMC you would report when checking in blah blah level 3300 IFR ?
Sure.
Would that not be technically incorrect and they may question your altitude?
Nobody will question it. In France they might... happened to me once; the ATCO asked a slightly sarcastic question to make me fly the correct semicircular level, without actually telling me what it should be
Normally the French are very laid back though.
And then when you were ready to request the transit of say SOLENT you would presuambly want to be at an IFR level correct with semi circular rule?
That's different because Solent is Class D. They will offer you a transit at a level/altitude
they prefer, say 4000ft, VFR or IFR as per your request. The moment you cross into their Class D, they will say "radar control service" and you read that back, and do as they tell you.
Note that you cannot get a transit of Solent (etc) above 5500ft because that is Class A and you can't go into that without a gold plated IFR flight plan filed previously via Eurocontrol.
if on a UK IMC to get the correct level to be accepted I take it would be illegal.
Not illegal in Class G, but pointless to test the algorithms in the Brussels computer when you just want to do a Class G flight
The Eurocontrol computer will normally not reject routings which are a series of DCTs e.g. EGMD DCT SFD DCT GWC DCT SAM DCT EGHH at 4000ft, but historically this used to be an issue especially if you picked a route segment which happened to coincide with an airway, and then the FP got chucked out for flying below the airway MEA.
What happens with such 'low level' flight plans is unclear, and I have never found anybody in UK ATC willing to describe it (one ATCO said it is commercially confidential) but basically as far as Eurocontrol are concerned it is a valid flight plan and they distribute it to the enroute sectors as normal (I have checked this with Eurocontrol personally, very recently) but there is some person in each enroute sector who looks at it, decides it is "below him" and bins it.
So an IFR FP like the one above will go to EGMD (who will note it), EGHH (who will note it), London Control sector(s) (who will bin it because it is beneath their responsibility; they only do "proper IFR") so really it is just the same as filing EGMD DCT SFD DCT GWC DCT SAM DCT EGHH at 4000ft
VFR. (which would go to EGMD, EGHH, and London Information which is the regional FIS unit).
This business of 'low level IFR flight plans' is a very tacky subject which is best avoided. Just file VFR. The fact that the whole flight will be in IMC, starting with an OVC002 departure, and ending with an ILS in OVC003, doesn't matter (in a Class D airport you will still need to go through the charade of an "IFR" departure).
And if you have an IR and really want to fly proper airways IFR then file for FL100 plus. Don't try marginal stuff like FL060... it is tempting fate because it does not look "serious" and is likely to get binned on "lack of seriousness" alone, and you need only a bit of that FL060 route to pop out into Class G and your IFR clearance will go in the bin right there, when you are airborne! I always file FL140 plus (have oxygen etc). One can always ask for a 'stop climb' when at FL100 and it is blue skies. Then of course you face the full brunt of the Eurocontrol route validation computer
I have sent you an email with notes on this.
As Johnm above says, Class G is really easy and relaxed in the UK.
Personally, I talk to nobody unless they can offer a radar service; Southend to Exeter that might be Southend, Farnborough East (anybody tried Gatwick Director lately; I have been handed to them a few times?), Farnborough, Solent or Bournemouth. Exeter.
It does beg the question as to why anyone would want to fly IFR, IMC, OCAS without at least a traffic service?
Because, when god made the sky, he made lots and lots of it
(I don't believe in god but you get the idea - midairs in IMC in the UK, since WW2, = 0 ). 99% of UK GA is VFR only so the skies are pretty empty anyway on not-nice days.