[Catfish]

The normal Diesel boats were too slow, and although the Walter propulsion made the later boats real fast, it was regarded as too much hazzle, without the pressure of war. The german test boat "V80" had reached a speed of 26 knots submerged, a speed that was only superceded in 1950ies by the U.S.S. Albacore.

Also England had made some mixed experiences with the german successors with Walter propulsion, be it the submarine propulsion of U-1407 (later H.M.S. Meteorite) or the rocket drive of the SR 177.

"During 1946 Meteorite carried out a series of trials under the guidance of Walter and his original team from Germaniawerft, Kiel. The trials raised considerable interest in the possibility of HTP as an alternative to nuclear power as air-independent propulsion and the Admiralty placed an order for two larger experimental Walter boats based on the German Type XXVI, HMS Explorer and HMS Excalibur, to be followed by an operational class of 12 boats."

High test peroxide is a dangerous material though, and as said before was regarded as too unstable to produce more than the two test boats.
With the US, Russia and then China developing nuclear submarines, England obviously decided to jump the bandwagon to gain experience and keep up with the superpowers. And soon as you have nuclear power plants and the technology, you can produce nuclear "fuel" yourself.

The MTU air independent propulsion would be the best idea for short range boats or difficult coast operations, but regarding independent missions thousands of miles away from home they cannot really compete with nuclear submarines. The 212 type can keep submerged for three weeks without snorkeling or surfacing, however its overall operation time is said to be not more than 12 weeks.