The Public Fundraising Regulatory Association (PFRA) is the voluntary self-regulatory body for organisations involved in this sort of fundraising. All its members follow the Institute of Fundraising code of practice. 

Fundraisers approaching you on the doorstep should wear an ID badge, and should be able to show you that they have permission to collect on behalf of a charity or group of charities. They should also be able to provide you with contact details of someone at a charity that you can get in touch with to verify this.

In the first instance you should contact the charity. Doorstep fundraisers should be able to provide you with the details of the right person to get in touch with.

The PFRA (the self-regulation body on this sort of fundraising) carries a complaints form on www.pfra.org.uk. The site also has a copy of the code of conduct that doorstep fundraisers should follow so you can check whether what you have experienced is in breach of this.