Hello & welcome,
I don’t see why you can’t defer installation untill you move into your new home. I woud speak to Motability of your wish to defer untill you move into your home with offroad parking. Alternatively (I would do it anyway), when Ohme or whoever gets in touch about you carrying out their survey, a simple reply suggesting you are moving into a home with offroad parking shortly and you will contact them regarding the installation once you have moved in.
As your probably aware, they wouldn’t install a charger on a property with on street parking, so its in your own interests to have the installation defered.
Yes you can charge with the “granny charger” in the meantime.