I think ss248 A (and B) may well not apply. These sections are designed to assist with a partition (or exchange) by joint owners. Condition A requires an initial joint holding and Condition D requires the consequential transformation into sole holdings. The sections do not deal with the reverse arrangement whereby sole owners transfer a property into joint ownership.
It seems likely that that each of ABC and D own their respective flats solely. Either these are flying freeholds or leaseholds (with the reversion perhaps owned jointly). The necessary planning compliance seems to involve the building ceasing to be sub-divided into separate dwellings. I query whether that can be achieved for planning law purposes while retaining the current property ownership configuration. Whatever that is legally, on the facts it seems very likely to constitute 4 separate dwellings. It is not clear to me whether it is possible to own in law a separate distinctive part of a single dwelling.
I imagine the most logical step would be for the 4 to own the property (the entire freehold or the existing leaseholds or one new lease after surrender of those) as tenants in common which would surely meet the planning obligation. It may possible from a property law standpoint for occupants of a single dwelling to be licensed, with some degree of exclusivity, to occupy distinctive parts of it e.g. like lodgers do, but this tenure together with a co-owners’ agreement regulating their inter-relationship may not be acceptable to them.
The procedures required if one co-owner wished to sell would need to be covered in it and apparently this operation would cause a part disposal for CGT by all co-owners of the interest to be carved out for sale to the third party buyer, plus further part disposals among themselves if the sale proceeds were then to be channelled to just one of them. That could not be avoided by use of s248A either, because under Condition D all the joint owners must each become sole holders of part. Giving one owner sole ownership of part with joint ownership of the rest left undisturbed is not covered.
Indeed trying to sell part of a property which must be used as a single dwelling for planning law seems fraught with difficulty (as in pre-contract enquiries) whatever the basis of its then current ownership structure in property law. Unless its planning status can be altered it may be that the sale of the entire building will be the only feasible future sale route. It would then be extremely prudent to have a co-owners’ agreement dealing with that prospect plus all manner of other matters including the death of a co-owner. A transfer into joint ownership now plus CGT liability and then occupying as mere licensees may be less attractive than a sale now (or soon) to a third party to fund the tax.
Lay clients have a right to instruct someone who may be inappropriate. Just as they are entitled to make home-made and £50 Wills. But there is a silver lining. Inevitably, later on, multiple competent advisers to the all the various separate (and probably warring) parties will have to sort it all out. Kerching!