The MTB / MGB was a good enough match for an E-boat, but, although it's powerful petrol aero engines developed far more power than a diesel of the same size/weight, it was much less efficient, far more flammable and extremelly noisy.
As Britain had little experience with high-power diesels, it was decided steam was the only way to go.
Twice the length of an MTB, it needed specialist builders, and absorbed scarce destroyer building berths, so 60 units was cut back to 9 - two of which were bombed and destroyed on the stocks.
To meet the specifications, half the previous power/weight ratio was needed - 14lb. per shaft horse-power. Only 50 tons were available for machinery, and although the boats with Foster-Wheeler boilers made the specifications, it was only the 2 tons lighter ( and 10% less powerful ) La Mont boiler that made the weight specification. As only one of each component could be installed, and there was no armouring, thesse ships were very vulnerable from a powerplant point of view - even machine-gun fire could knock them out.
The armament was greatly strengthened very soon, and 3/4 inch plating to the machinery spaces helped to add some survivability.
In 1944, they were all converted to fast minesweepers to support the invasions.