Cessna 172 IFR Airways Flightplan..?
 

Hi All, as a VFR pilot, I know nothing about IFR flightplans, but some of the professional members of our group fly to the continent. So I was wondering if they would be allowed to fly in the airways with our C172. Obviously they would have to decline any ATC requests to climb to FL300.
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Yes,if the aircraft has IFR avionics and the crew an IR, no problem. Although on the Continent, France for instance, you can fly VFR in Airways, with suitable weather. Recently Cannes in a C182 VFR. FL 090/120 all the way.

Sure you can. It isn't always easy because traffic issues around, for example, Heathrow may cause London to have to descend you out of controlled airspace, you you may end up in IMC in class G without any radar service or rerouted with vectors instead of your flight plan. But that isn't exclusive to a C172 either. Typically to fly to the continent you'd be at FL080 to FL011 by filing at the lowest available levels. No one is going to ask you to climb to FL200. You'd be in airways (class A) in the UK / IFR only. unless you get dropped out into D or G. Elsewhere in Europe, in class E you'd be VFR/uncontrolled, mixing with IFR traffic which is controlled. ( hence the comments about "being on airways " in europe, but it isn't really).

Re cruising levels - quadrantal is gone and UK now follows semi circular, as does pretty much everyone else. From the UK CAA website :The semi-circular level system replaces the quadrantal system and brings the UK into line with ICAO standards applied elsewhere around the world. VFR and IFR aircraft are allocated different levels to fly at:

• IFR flights use whole 1000's of feet (e.g. 1000, 3000 etc. when flying eastbound, and 2000, 4000 etc. when flying westbound)
• VFR flights use the intermediate 500 ft. levels (e.g. 3500, 5500 etc. when flying eastbound and 4500, 6500 when flying westbound)

For VFR flights, compliance with the cruising levels remains good practice but is not mandatory.

Or they may transfer you to one of the 3 radar frequencies operated by Farnborough for low level IFR and VFR traffic under the LTMA.
Back in 1969, I remember a guy in a Cessna 150 filing IFR from either Luton or Stansted to Brussels requesting FL90. As it was for a departure at about 0200 UTC there wasn't too much other traffic about so it should have been possible, but the controller (JR) who issued procedural clearances obviously didn't agree with this and issued a clearance at FL90, but to maintain not less than 150 kts, clearly impossible in a C150.