Anything other than a victory would see Jos Buttler's men knocked out, with Scotland joining already-qualified Australia in the second-round Super Eights before the two countries meet later Saturday in St Lucia.
England are currently two points behind Scotland in Group B but with a superior net run-rate that will be the tie-breaker if both teams finish level on points.
Title-holders England inflicted an eight-wicket thrashing of Oman on Thursday as they chased down a target of 48 in just 19 balls -- the largest win in T20 World Cup history in terms of balls remaining.
But the game with already-eliminated Namibia is England's last in a Group B where their opening match against Scotland ended in a washout before they suffered a convincing 36-run loss to Australia.
Even if England beat Namibia, they could still be knocked out should Scotland achieve a stunning upset win over Australia or if that game ends in a no-result.
England, in a game delayed by three hours, made two changes to the team that hammered Oman, with left-arm paceman Sam Curran and 'death' bowler Chris Jordan replacing express quick Mark Wood and spin-bowling all-rounder Will Jacks.
Teams
England: Phil Salt, Jos Buttler (capt/wkt), Jonny Bairstow, Harry Brook, Moeen Ali, Liam Livingstone, Sam Curran, Chris Jordan, Jofra Archer, Adil Rashid, Reece Topley
Namibia: Nikolaas Davin, Michael van Lingen, Jan Frylinck, JP Kotze, Gerhard Erasmus (capt), JJ Smit, David Wiese, Zane Green (wkt), Ruben Trumpelmann, Bernard Scholtz, Jack Brassell
Umpires: Adrian Holdstock (RSA), Langton Rusere (ZIM)
TV umpire: Rashid Riaz (PAK)
Match referee: Ranjan Madugalle (SRI)
