Union funds should not, therefore, be used for donations to outside causes.
Income generated by a union's commercial activities is treated in the same way as other union funds such as block grants from the institution - ie commercially generated income is also subject to ultra vires principles.
These could be said to be primarily social and recreational, and therefore furthering union members' education (in the wide sense), and therefore falling within the union's charitable objects. The raising of funds for an outside cause could be said to be a secondary purpose of such events.
There are a number of rules to bear in mind when holding benefit events:
It is important that you charge in this way from the point of view of PPL licensing as well as ultra vires. NUS is presently arguing in court that student unions are entitled to exemption from having to pay PPL because they are charitable and because the proceeds of their admission charges to events are applied solely for the purposes of the organisation. If the admission charge for an event goes partly towards an external cause, this external cause will not be one of the purposes of the union, and so the union would lose its exemption and have to get a PPL licence to cover the event. If however a donation to the external cause is requested separately, this does not count as part of the 'admission charge' and so the exemption should still apply.
Note however that the whole issue of student unions' exemption from PPL is uncertain at present. It is due to be considered by the Court of Appeal shortly and NUS will advise on the outcome as soon as it is known.