But 20 knots = 480 nautical miles per day. 25 knots = 600 nautical miles per day. And that's carrying all your engineering, logistics and other support facilities with you.
This isn't the way the Government do there sums but....
Estimated cost of CVF = £15 000 million
Predicated lifespan of CVF = 50 years
Therefore cost for two ships per year = £300 million.
Cost for ship per year = £150 million.
Which is cheaper than a frigate, and less than three Typhoons.
Something to think about.