when you say "without a key" I think you mean a bolt. You can get concealed bolts that are useful in these situations. Either a mortice bolt operated with a thumblatch, like a bathroom bolt
https://www.ironmongerydirect.co.uk...ase-57mm-backset-satin-stainless-steel-321796
https://www.ironmongerydirect.co.uk/search?query=bathroom turn
or the very common mortice rackbolt, operated with a removable key or a small permanent knob (these are lower quality than the bathroom lock, and the star wears out).
https://www.ironmongerydirect.co.uk/search?query=mortice bolt
In your case, you would have no keyhole or thumbturn on the garage side of the door, only on the house side, so an intruder cannot open them. It is usual to have one near the top of the door and one near the bottom, to supplement the mortice lock you have near the middle of the door.
Your mortice lock should be a 5-lever British Standard Deadlock or Sashlock (the sashlock includes a handle operated latch). You wil not find a stronger or more secure lock. The red-cased Union is good value.
https://www.ironmongerydirect.co.uk...-case-57mm-backset-sat-stainless-steel-570978
I presume that your door to the garage is a fire door, which is solid, heavy and 44mm thick. These are quite strong. If it is fitted with fire hinges they should be thick and strong, if not, upgrade them. Add hinge bolts if you like.
https://www.ironmongerydirect.co.uk/search?query=hinge bolts
if the door is a lightweight internal one, throw it in the skip and buy something better.
You would still benefit from an intruder alarm with sensors on the garage doors and a PIR inside, because once a burglar has got into your garage, he can work away unobserved to make an entry into the house.