What's the cheapest way to send out 10,000 posters to businesses in the uk?

We are a small company and need to send out an A4 poster to nearly 10,000 selected businesses in the uk for them to display. It seems the cheapest way to do this is to buy prepaid 2nd class envelopes from Royal Mail and this will cost in the region of �4k. As this is a one off we have been advised against franking machines as the saving wouldn't be worthwhile? Is this correct? Any helpful suggestions would be warmly welcomed.