BPP Mock 2 Q3 capital allowances

It is because of note 9 in the question, which does not say that it is being sold - you are told the value as a continuing operation (i.e. the value of all later flows). Therefore the answer has continued to calculate WDA for the 4th year i.e. 25% x 1856. However, the note could be read in different ways and you would still get the marks if you had calculated the balancing charge instead.