If interested plesae look for flights in travel dates till end of June 2015. (except blackout period of your outbound flight is 19.3 - 3.4. Return flights are not permited between 3rd and 20th of April). Booking is possible till 20th of January 2015 or until sold out. The easiest way to check your connection possibilities is through Skyscanner. This tool gives you quickly a rough idea which low-cost airlines are connecting your home town.
Tip - Booking a stop-over in Istanbul is available and in fact even greatly decrease the fare with Turkish Airlines. Take this opportunity to visit this beautiful city on your way to Africa! Please refer to below booking samples if you are interested in this option..The simplest way to look up the cheapest travel dates is to check here using +-3 days searching tool. If you are looking for an accommodation you may check here to compare hotel rooms through various booking sites.
Booking sample
UK - Africa from Ł342 (directly with Turkish Airlines)
UK - Nairobi from Ł305
UK - Addis Ababa from Ł318
UK - Dar es Salaam from Ł311
UK - Istanbul - Nairobi - UK from Ł283
UK - Istanbul - Addis Ababa - UK from Ł295
UK - Istanbul - Dar es Salaam - UK from Ł289
Booking sample fo cheap return flights from UK to Africa for Ł283 here. in this case you would be leaving from London to Istanbul on 10th of May. You would then fly from Turkey to Nairobi on 13th of May. Finally you would be returning back from Kenya to London on 29th of May 2015.