It will potentially depend on the exact country, but most countries in African are NOT considered "clean" from a security perspective. That means that as you've suspected, you will most likely need to go through the transit security area before being let into the departure area.
Depending on the time of day, this can take anywhere from a few minutes to at least half an hour or more. If you are in business class or have Star Alliance Gold status it will generally take less, but even that's been a bit hit-or-miss recently.
55 minutes from a non-clean country is going to be tight, especially as many flights into Istanbul airport nowadays end up using "bus gates" where you have to catch a bus to the terminal - adding further delays even if your inbound flight is delayed.
Personally I would look for a longer connection if possible. If you're headed to London it's worth considering alternative airports as well (eg, Gatwick rather than Heathrow).
If there's no better alternative, 55 minutes IS doable (they wouldn't sell you the ticket if it wasn't), however you do run a real risk of not making your connecting flight - but at least as you're on a single ticket it will be Turkish Airlines' responsibility to put you on a later flight.
If you're doing the same itinerary in reverse it's a little different as the UK is considered clean, so they will allow you to deplane directly into the departure terminal (presuming you already have your connecting boarding pass).
