How's about a shed in the garage?....In other words a smaller enclosed space to heat, rather than heating everything else in the garage.
Sealing up draughts around garage doors is well worth the effort, but with our recent renovations I treated myself to a Hormann Sectional Insulated door, brand new to a bespoke size. This has rubber seals all round, and double skin insulated sections. Folks sell these off relatively cheaply when they turn a garage into an extra room, a more common conversion these days in the UK. Sod's Law mine was an odd size, but thanks to YouTube, fitting it myself was easy and I saved myself the best part of a grand's fitting charge. So pleased, I got a cheap second hand one in a standard size (on eBay) for £150. This is fitted at the back of the garage, so I have access straight through the garage to the back garden. Both are motorised, real impressive.
The garage door suppliers are in essence booking agents, the new door was delivered directly from Hormann's depot in Leicester. Other similar and cheaper door brands were available, but buying one from Latvia or Bulgaria with PayPal just didn't give me the same confidence.
Swarfy.