In general, if you want to use a building as rented accommodation set out in flats, and it's not currently used for that purpose, you'll need planning permission for a change of use.
If you're buying a building that is currently used to provide rented accommodation set out in flats, but clearly wasn't built for that purpose, then you will ask the vendor to produce planning permission for the change of use — you don't want to buy the building, only to find that the current use is an unauthorised development and you have to stop it. The vendor can either (a) give you a copy of the planning permission, or (b) given you evidence that the current use has been carried on since before 1963. In the latter case that means the change of use occurred before 1963, at a time when planning permission wouldn't have been required.
But this is only important in relation to the current use. If the property used to be let in two separate flats, but is currenty a single property used as an owner-occupied dwelling, it doesn't matter that, in the past, it was let in two separate flats, and it doesn't matter whether that use predated 1963. Because it's not the current use, if the purchaser wants to let it in separate flats, that will be a change of use and he will need planning permission. Knowing that it was set out in two separate flats before 1963 is no help to him.
Also, how does pre 63 use become abandoned? Is the very fact that it has been used as a family home for the last few years proof enough of pre 63 abandonment?!
When a particular use has been abandoned can be a bit of a grey area. Has it been abandoned, or is this just a temporary break? How long can a "temporary break" last, anyway? But this case doesn't look like it's going to fall into a grey area. It's not just that the old use ceased, but a new use (as a single owner-occupied dwelling) commenced. Plus, almost certainly physical works will have been carried out on the property to reunite the two flats into a single house. These factors would point to the old use having been abandoned. You would also look at factors like, how long has ti been used as a single owner-occupied dwelling? Obviously, the longer this has gone on, the harder it is to say that the old use has not been abandoned. Without knowing all the details it's impossible to be categorical, but I think you'd need quite unusual facts to argue that converting the house back into a single home and using it as your own residence isn't an abadonment of the former use of letting it in flats.
