We don't have actual reports from travellers going this route, so you'll be the pioneer!
My first look would be at DAS Air Cargo (part of ANA, which is UK based). They have a website and scheduled flights from Accra. E-mail: firstname.lastname@example.org
or call them in the UK +44 (0)1293 547737. Their website is at www.dasair.com.
My next contact would be British Airways Cargo, probably more expensive than DAS Air, though. http://www.baworldcargo.com/stations/index.jsp
As the topic of flying from Ghana came up last year, there is also contact info for several carriers in Ghana at the following link.
I'd be more concerned with the arrangements if you were flying the bike INTO Africa (or if you were going by sea), than with flying OUT of Africa to UK. When you get to the end of your trip, just go out to the airport, find out which airlines fly direct from Ghana to UK (very important!). Ask the airline cargo office to recommend an agent if they can't deal with you directly, and sort it there.
Good luck with your trip, let us know how you go, and don't forget to fill out the Shipping form when you return so that other travellers can benefit from your experience.
"It matters not what goal you seek
Its secret here reposes:
You've got to dig from week to week
To get Results or Roses."
'One world, two wheels'