If I was a wannabe terrorist I think it'd be pretty pointless to start from the UK anyway when it'd be much easier to start my journey at pretty well any west African airport.
Over the past few years I have flown regularly into and out of Lagos, Port Harcourt, Pointe Noire, Malabo, Angola, Port Gentil, Abidjan and Douala airports. Almost all of these have no decent security. Quite often the x-ray machines don't work. In almost all cases, the only question you will be asked by "security" is "what do have for me?" Pointe Noire was by far the worst for this, with Malabo a close second.
On arrival in Douala after a short flight from Malabo, I was handed a "security checked" sticker to put on my suitcase for the onward flight to CDG, with no checks being carried out at all. At most airports, the agent who would pick us up and take us to the airport was a buddy of the security staff, resulting in no checks on checked-in baggage.
Most European airline do have their own security staff on flights from west Africa who make checks on hand baggage before boarding but this often done in a rush on the tarmac at night and is clearly far from ideal. Some airlines, e.g. BA, also insist that checked-in baggage is identified on the tarmac by the SLF it belongs to but that doesn't necessarily mean it's safe!
Why would any terrorist start from the UK when he could blow up a plane full of Europeans and Americans on their way home from Africa?