The money wasn't spent just to reduce the Eurostar journey time. That's the headline item because the press & tv don't bother to give full reports.
You should read the specialist railway magazines for the full story.
Basically:
The Waterloo terminus was not capable of expansion so further trans-European services could not be accommdated.
St Pancras is able to take more trains so more destinations will be on offer in due course.
Waterloo is congested so the release of the Eurostar terminal to domestic services will improve the train performance and punctuality at the station. (Don't be tempted to say "oh yeah", it will make a difference).
The old route to Waterloo is not able to accept trains of the larger European loading guages. Now it is possible for a foreign high-speed train to make its way direct to London. (More future services again, when they recover from the shock of getting this one right).
The Waterloo lines are fed from third rail DC power supply. This meant that the Eurostars had to be fitted with retractible 'shoes' for power pick-up as well as roof-top pantographs. This is an expensive complication for maintenance on the trains. They have now been removed. The low voltage DC also severely restricted the England-side speed.
St Pancras station itself is a Listed Building and needed millions spent on it to stop it becoming dangerous. As this money was going to be spent on the building anyway it made sense to spend relatively little extra and develop the railway station itsef.
The high-speed line now goes north of London thus enabling future through-freight (some at the enhance loading guage) trains from what's left of the industrial North to pass direct to Europe.
There's more, but you can see that the money wasn't just spent to save a few commuters 20 minutes. Even if there was a smaller time saving most of it would have had to be done anyway.