You are assuming that UKF cap slower in preview when they can actually cap faster by producing a Ro.E.
Vipper is actually right on this I did some testing.
The chosen map was crossroads. I did a regular 4 section build on the live patch, a 4 section build on the test patch, and a 2 section, sapper, Section build on the test patch. The requirement was to cap one half of the map and build one sandbag on the fuel point.
The 4 sections build on the live patch capping speed was 2:05
The 4 sections build on the test patch capping speed was 2:12
The 2 sections, sapper, section build capping speed was 2:03. At 2:12, I capped 2 more standard territory points than the 4 section build on the test patch.
Keep in mind all timers were started at 0, I made sure to edge cap as well.
This means that theoretically, it is true that a brit player will have more map if they go for a sapper after 2nd section because of the reduced cost and build time of the sapper allowing it to come significantly faster than the 3rd section would in a 4 section build. A Sappers squad will definitely allow the brit player to sustain map control early if it just came down to capping ofc.
Whether or not that holds true when another player is contesting it is another story. Will need some case sample games to determine that.