Drywall – How to plaster a hole in the wall on top of thermalite

There are a few possibilities; without knowing more about the layout of the house makes it harder to say for sure.

The internal wall in the garage might be constructed in normal blocks for fire performance reasons. (Without looking, I'm not sure about the fire performance of thermal/lighweight blocks). Also, normal blocks might have been used for structural reasons - for example if the wall is load-bearing (normal blocks generally have a higher compressive strength than thermal/lightweight blocks).

Likewise with the bathroom and en suite - there might be a difference for structural reasons.

Or there might be no firm reason for the difference at all, other than the builders using what they had available - it's not unusual for builders to use what they have as long as it meets any specific requirements for the particular element of the structure.

What I have done on my 1909 house (I am in the U.S.) in the same exact situation is clear a wider area of the lath and plaster and replace it with drywall. To do that, you will need to add some padding over the studs because the lath&plaster is thicker than 1/2" -- what I did was pad it with 3/8" plywood, then drywall over plywood to achieve approximately an even joint between the new drywall and the old plaster. Then you tape and sand the joints like you would with regular drywall, prime and paint it later.