disposable income is exactly that. If the OR has agreed to the income / expenditure then there is absolutely no reason at all that you don't save for a car / bike / holiday etc etc .
yes it is an asset but not at the point of the Br and also not as a windfall received during the br.
Once the or has agreed the I&E the disposable income is agreed and therefore yours. You may remember that the OR discussed life insurance policies or accident , sickness & redundancy policies with you, pr even SKY movies.
Whilst these are not an authorised expenditure the OR cannot decide what you should do with the agreed disposable monthly income.
Obviously putting some aside for emergencies is good.