Nimbus,
A 'quadrantal' is a cruising level based upon 500 ft separation and on 1013/29.92 . Hence it is basically similar to a semi-circular cruising level (except that there are 4 quadrants rather than 2 sectors), but because our UK GA traffic often flies around above our low transition altitude (3000 to 6000 ft depending upon where you are!), IFR traffic above 3000 ft amsl must fly on 1013 and fly at an appropriate cruising level. So, for example, if you're flying on 120 deg you need to be at FL35 or 55 or 75 etc., not at an altitude based on the local 'altimeter' as you would in the US. Although there is no actual legal requirement to talk to anyone in IMC, it would be plain daft not to. So if not under Radar Control, you take either Radar Advisory Service (only available under IFR) or Radar Information Service; the main difference between RAS and RIS being that under RAS the radar unit will attempt to provide the required IFR separation from other traffic, whereas under RIS you will be told of other traffic nearby and it's then up to you to manoeuvre (or maneuver!) appropriately. Why would anyone want RIS under IFR? Because to provide IFR separation to the extent which the ATC unit is required to can involve extensive vectoring; pilots who can 'walk and chew gum at the same time' or who can fly and think at the same time under IFR often prefer just to take their own avoidance action based upon the situational awareness provided by the radar unit. You might be quite happy with "Traffic left at 10 o'clock 2 miles, heading away" under RIS and choose not change your heading, but under RAS the controller would have had to have vectored you earlier to provide greater separation!
Very simplistically, the IMC Rating is a basic instrument rating dsigned to equip UK pilots to cope with flight in IMC, but it is not the same as the Instrument Rating which in the UK requires far greater levels of ground and flying training plus testing in a comprehensively-equipped aircraft (and is much, much more expensive to obtain!). The IMC Rating has more restrictive take-off minima and recommended landing minima than an IR and it does NOT permit flight in Class A airspace. So if there's a layer of stratus with poor visibility underneath, rather than scrape around in marginal VFR underneath, with an IMC Rating you can get a radar service and climb to VFR on top, cruise to your destination, letting down again with a radar service to fly an ILS or GCA down to a 500 ft QFE DH or an NDB to 600 ft MDH (higher if the procedure minima dictate). Used within its limitations for the purpose intended, the IMC Rating is a very good rating for the average UK private pilot to obtain, but it is not a substitute for the PPL/IR. Also, if you're just going from A to B (remaining outside controlled airspace), even if you think that you'll need to fly in a bit of IMC en-route (at which point you'll be flying IFR), you don't need to file any sort of flight plan in the UK! No flight plan needed just to climb up through a bit of cloud to VMC on top!!
[This message has been edited by BEagle (edited 28 January 2001).]