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)