When you have decided to become an official group/charity you will need to set up an organisation
Small Charitable Organisations: If you intend your organisation to have an income of less than £5000 you can adopt the charity commission small constitution and do not need to register with them. unless you would like to be incorporated in which case you would chose a Charitable Incorporated Organisation which will have to register with the charity commission regardless of income level.
Larger Charitable Organisations (income over £5000): If your organisation will have an income of over £5000 you will need to register with the charity commission. You can either be an Unincorporated Registered Charity or CIO (Charitable Incorporated Organisation)
For more information on these and Company structures go to: http://ervas.org.uk/setting-up-your-organisation/