Callsign Kilo,
From your post, it seems there are some
very large gaps in your knowledge. This is a pity, and my guess is that the root of the problem is that you were taught to pass the IMC test, rather than fly in IMC - probably by someone who believes the rubbish that an IMC rating should only be used to get you out of trouble.
To answer your questions as fully as I can on a forum like this:
However where I am unlcear is how does flying under 'IFR' work with an IMC rated pilot?
In the UK, you may fly IFR outside controlled airspace, using any combination of approved navaids. Probably the most common navaids for en-route navigation are VOR and GPS. You must comply with the rules of IFR flight - that is being 1000' above the highest fixed object within 5nm, and following the quadrantal rule if above 3000'. Assuming your aircraft has minimum level of equipment as required by the ANO, you can ask for, and may get, an IFR crossing of Class D/E controlled airspace such as control zones or control areas, but most of your en-route flying will take place outside of controlled airspace, where you should make use of whatever radar services you can get if you are in IMC.
I know the rating is restricted to the UK, however in what way is it practiced? Can you file IFR flight plans, even though you cannot fly in airways?
You can file a flight plan for
any flight. You
must file a flight plan if you are crossing the UK FIR boundary, and also if you plan on flying on airways. You are recommended to file a flight plan if flying in remote areas or over large bodies of water. You can file a flight plan for any other flight if you wish. As an IMC-rating holder, you do not need to consider flight plans any more than you did as a basic PPL holder. What I mean by that is that, if you are flying to France, you would have to file a flight plan whether you plan to use your IMC rating on the British portion of the route or not. On the other hand, if you are flying from Cranfield to Shoreham, you do not have to file a flight plan if you are flying VFR (although you may if you want), and you also do not have to file a flight plan if you are flying IFR (although, again, you may if you want).
Can you upgrade to IFR from VFR if the weather starts to get below VFR minima or if you need to decend through cloud to get into an airfield?
Yes. Outside controlled airspace, you can switch between IFR and VFR as often as you like, without having to inform anybody (although you might like to ask for, or cancel, a radar service). Inside controlled airspace, your clearance will be either VFR or IFR, and you must inform ATC if you wish to change flight rules.
Having said that, with limited experience, trying to plan an IFR route (or even half a route) on the hoof is not to be recommended. If there is even a slight chance of the weather being IMC, I would suggest planning the whole flight as IFR right from the start, then downgrade to VFR during the flight if appropriate. As you get more experience, you will become more comfortable with the idea of upgrading to IFR part way through a flight with minimal prior planning.
What are the DH/MDH minimas for an IMC holder. For my test I was told to add 500 ft to the published minima + the Pressure Correction error for my aircraft. Does this still apply?
I hope that's a typo. The recommendation is to add 200' to the published minima (plus PEC for a precision approach), and then to round this up to a minimum of 500' above ground for a precision approach, or 600' above ground for a non-precision approach. (If you prefer to fly instrument approaches on QNH - probably the most common scenario - rather than QFE, this takes a little bit of maths.) This is just a recommendation, but one I would suggest you stick to until you have more experience. (In any case, if you are flying a single-engine aircraft, would you really want to be flying if the cloudbase was below 500'???) This is described in detail in both the AIP and Trevor Thom book 5.
However, Fuji is wrong when he says "Your landing minima are the same as the minima for IR holders". [Edit to note that Rustle got in before me in spotting this.] Although the increase DA/MDA I've described above is just a recommendation, the increased visibility for IMC ratings vs IR is
mandatory. It is 1800m for both take-off and landing. In the case of landing, what this means is that, on an approach, you may not descend below 1000' agl if the vis is less than 1800m. However, once you have descended below 1000', if the visibility subsequently deteriorates to below 1800m, you may still continue the approach, and you may land if you have acquired the necessary visual reference before DA or MApPt.
I hope that helps, but it really does sound, from your post, that you have been left with big enough gaps in your knowledge that you really need to spend some time with a competent instructor and/or experienced IMC-rating holder before attempting to use the rating in anger.
Good luck in acquiring the necessary knowledge, and using your rating's privileges!
FFF
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