Marketing via Post

Hi I am just finalising my application for my practising certificate. I am planning to move from practice into industry, but also try to obtain a few clients for both bookkeeping/payroll and accounts/taxation services to act for in my own free time.

As it is for now only going to be a part time venture, I want to keep costs to a minimum, including marketing and advertising. I am aware that regulations are in place to not allow contacting and marketing to potential new clients via email or over the telephone without a prior 'opt-in' of the client.

However are the regulations different for sending marketing materials to a businesses address via post (or delivering myself)? After all I have a stream of marketing leaflets and letters every week sent though my letterbox. If it is allowed, are there certain promotional items which can and cannot be sent? If I can I would be planning to send out a mixture of leaflets, business cards and letters which can allow me to present myself in a more detailed way.

Thanks in advance!