Every postal ballot has a unique id, every voter has a unique id. The corresponding numbers list matches the two.
So every single voter can be connected to their vote. In addition every single postal ballot is logged on the marked postal ballot list.
tractability is not an issue
The issue is that the corresponding numbers list, and the ballot papers are sealed for one year, and can only be opened with a court order. So it has to be pretty much proved the electoral results in one constituency are incorrect, before a court order could be obtained