Need a new passport but can't send old passport

Hi, my passport is not near expiration but I only have 9 blank pages left and in 2 months I'm going to embark on a big trip across Africa and gov.uk says I will need over 20 blank pages for all the visa stamps. This I suppose means I need to renew my passport, the only issue is in about 3 weeks I'm going on another trip where I need my passport and I believe you need to send off your old passport to get a new one? Does this mean my only option is the fast track service when I return? Is there a way to get a new passport without sending off the old one? Anybody have experience of getting a new passport sent out whilst abroad? Many thanks, Jonny