If a person holds an IMC rating, can you file an IFR flight plan?
Yes. (You don't, in fact, even have to have an IMC rating to file an IFR flight plan or fly IFR in the UK outside controlled airspace, but those without are limited to an in flight visibility of 3 km.)
If so how do you notify ATC that you are not Instrument rated and cannot accept certain clearances (ie into ClassA)?
I suggest you make sure that the filed route/level is consistent with remaining outside class A. Apart from that, all you can do is notify the controller if you feel it is necessary or appropriate.
Does the IMC rating allow the holder to fly in the clouds or is it purely for a transition to VFR on top?
Yes, it allows flight 'in the clouds' provided the visibility below cloud for take-off and landing is at least 1800 m.
How can you learn to fly under IFR in IMC safely in 15 hours? That's the toughest question...
BTW sanjosebaz's quote is at least 5 years out of date. Try
The ANO 2000 Schedule 8, in particular Part B - Ratings for the letter of the law.