British Airways Nigeria fly six times between London and Abuja, and twenty one times between London and Lagos every week. They have between 267 and 291 seats available per flight.
The airline operates direct flights between the UK and Nigeria but ticket prices are not cheap. Expect to pay between £600 - £900 return depending on the season. Ticket prices get more expensive during Easter or Christmas.
From the United States, BA flies to Nigeria via London Heathrow. Say from JFK -> London -> Lagos/Abuja->London->JFK. Total cost of such flights can be up to $4,500.
They want you to check in three hours before departure and baggage allowance is two bags of 32kg each.