There are a few big names offering franchises to bods with their own vehicles. One is 'Business Post, and another is 'APC'. Business Post are already big in Canada, Australia and New Zealand and expanding rapidly across the UK.
Be careful though, because the small print of many of the contracts leaves you wide open to have the p1ss taken out of you. Having said this, you could do worse than to investigate how they operate and adopt some of the 'best practice' policies they use.
Insurance could be difficult, as well as restrictions on what you can carry (any Hazmat experience you've got will help).