Hi,
from what you have said,i think the easiest way to give you more/better insulation would be to fix new Gyproc Thermaline Super boards over existing garage ceiling (of which incidently should already be firelined or double-boarded to building regs).
These come in various thicknesses ranging from 30mm to 90mm thick,and have a very good fire rating to Class 0 on both faces.

The thicker you can go to the better your insulation will be but obviously there's a difference in price as well.

In my opinion,using foil-backed boards won't really make much of a difference as they are designed for vapour control and not insulation.

Although it may not the cheapest method you are recommended/suggested to do,using the Gyproc Thermaline Super system will certainly make a significant improvement & noticeable difference to your insulating needs & will be money well spent.
Here is a link on the product itself for more information.

http://www.british-gypsum.com/products/plasterboard___accessories/
gyproc_thermal/gyproc_thermaline_super.aspx

Hope this has been of some help to you.

Carl.

If you cant lift floor then the best way would be to over board your garage ceiling with foil backed plaster board with skimmed ceiling, all joints taped and sealed around edges.
It should be double boarded, fire regs.

personally i would have put 100mm kingspan between your floor joists,this is a solid insulation and comes in sheets roughly 2.4m x 1.2m.Then i would have put foil backed boards on your garage ceiling before plastering, and obviousely a good quality thick underlay will help.Hope this has been helpful to you.
regards
HomeCare Team