100% agree with John about the floor.
I insulated my (detached) garage floor with 25mm jablite and then 18mm caberfloor ontop. Very simple to do, cost about £150 and makes a massive difference.
I also dot and dabbed Gyproc Thermaline Plus 40(30mm insulation + 10mm plasterboard) to my single skin walls. Took 2 days, and about £300, but it now takes very little heating to keep the garage at a comfortable temperature.