An unincorporated association is a membership organisation. It can be whatever its members want it to be, and carry out whatever activity you choose. It is the easiest, quickest and cheapest way for a group to set itself up. Unless there is a membership fee, you are not even obliged to keep a membership list - anyone who is entitled by your rules to be a member can simply turn up and take part.
You do not have to seek approval of any kind before setting up. Nor do you have to register with any regulatory body, though if your group has charitable aims and an annual income above £5,000 you are required to register with the Charity Commission. For information about charity requirements and governance, visit their Charity Commission website http://www.charity-commission.gov.uk/.
