The UK IMC rating may be 15 hours of a course but the actual IMC / simulated IMC time is very small. Contrary to what someone said earlier it does very little to prepare the pilot for IFR enroute flight
Precisely what preparation would you say is needed for "enroute flight"?
Let me try to enlighten you. You turn the yoke so the AI is level, and you push it and pull it (with trimming for the more advanced pilots) to maintain altitude. You do what ATC tell you re headings/levels etc. You might be asked to hold (extremely rare) but I did holds in the IMCR so that's OK. You can fly any IAP too
The enroute element is basically VFR enroute planning but above MSA and in IMC.
which is what obstacle-aware flight is, surely?
In airways flight, the obstacle clearance enroute is done by airway MEAs / ATC MVA etc. So that is easier. IMCR flight is done without this system/ATC support, which is why pilots are taught about MSA.
There is no SID / STAR training despite the fact that in the UK, the privileges entitle the pilot to Depart on a SID, fly along an ATS route and compete a STAR with IAP at the destination.
Not in practice.
Firstly, there are hardly any (any??) SIDs in the UK whose join to the enroute waypoint is below Class A, so an IMCR holder could not usefully depart on one of these.
Secondly, if a SID is assigned, it will be handed out in the departure clearance, and if the filed flight plan was filed below Class A then the flight control officer (or whatever they are called here) isn't going to issue a SID. The only way to get a SID issued is if one files a legit Eurocontrol airways (which means generally FL100 plus) flight plan, which for an IMCR holder would be an illegal flight because there is almost no way to file a real IFPS flight plan without hitting Class A. I know of cases where this was done as an honest mistake and it caused havok, and I think measures have been implemented by London Control etc to avoid this - by them washing their hands of such low level flight plans. So no SID will be assigned to start with.
Thirdly, there is no STAR in the UK whose terminating (entry) point can be reached below Class A. Maybe something can be hacked which is flyable wholly in Class D but one would really have to work on it, but one would never be sure what IFPS/LTCC did with the flight plan, so the usual airways implied IFR clearance could not be assumed, so why bother filing such a contrived thing in the first place?
On the IMCR, one flies a mixture of CAS and OCAS, DCT as much as possible, and one joins to airports as directed by ATC.