The character of Dieter Koehler appears to be based on that of Hans Multhopp, a German aeronautical engineer/designer, who was brought to the U.K. by T-Force in 1945.
The biggest error of this series is the assertion that Britain needed German jet engine designers after WWII; in fact, by the end of the war, British companies such as Rolls Royce had surpassed the performance of the notoriously unreliable German jet engines. On inspecting the German jet engines in 1945, after the fall of Germany, Rolls Royce engineers were very unimpressed with the state of their jet engine development.
A more correct version of the plot should have focused on the number of German aeronautical engineers that came to work in the U.K., either willingly or taken using T-Force as shown in this series, and included engineers such as Dietrich Küchemann, Johanna Weber and Hans Multhopp. Multhopp did in fact design a supersonic jet aircraft for the British at the RAE - however, as soon as it reached the prototype stage, Multhopp was taken off his own project, as the project became 'top secret', and he was classed as an 'enemy alien' working in Britain, he couldn't work on a top-secret program.