The mental performance effects most likely experienced from Berocca are from an ingredient you didn't list: caffeine. In the US, you cannot buy Berocca without guarana/caffeine.
Caffeine is a stimulant and has some cognitive effects due to that. It could be said that all of Berocca's mental performance claims are true due to the caffeine (e.g., alertness, reduce fatique, mental performance, etc). In addition, there was at least one study that showed that eating a better diet improved mental performance.
Caffeine and mental function - Caffeine may have an effect on mental function, particularly for older people, but perhaps not on memory other types of cognitive performance.
The Journal of Nutrition study isn’t the last word on the subject of
caffeine and memory. It showed that people—particularly those who were
ages 70 and over—who took in more caffeine scored better on tests of
mental function, but not on memory tests or other measures of mental
Source - Caffeine and a healthy diet may boost memory, thinking skills; alcohol’s effect uncertain
Caffeine, cognition, mood, memory - Abstract from a study on the cognitive effects of caffeine on memory and mood:
Caffeine facilitates learning in tasks in which information is
presented passively...caffeine appears to rather improve memory
performance under suboptimal alertness conditions. Caffeine has also
been reported to prevent cognitive decline in healthy subjects.... In
conclusion, it appears that caffeine cannot be considered a 'pure'
cognitive enhancer. Its indirect action on arousal, mood and
concentration contributes in large part to its cognitive enhancing
source - Is caffeine a cognitive enhancer?
Diet and mental performance - The same Harvard article from above also studied the effects of diet on mental performance. As might be expected, people with a healthy diet performed better:
The study also looked at the connection
between diet and mental performance. People who ate foods with plenty
of healthful nutrients had better attention and memory than
participant with poorer diets.
Physiology of caffeine on the brain - Lastly, for a longer explanation into the physiology of how caffeine affects the brain, here's an answer from Quora (that people probably won't like, but whatever). It also has many other references if you're curious about the physiology of caffeine and the brain. Overall, the reason caffeine affects the brain's mental performance is because it provides for more energy sources, in addition to glucose.
In case you didn't notice the caffeine:
Includes Guarana and caffeine to support mental sharpness and
B-vitamins to support physical energy by helping to convert food to
Berocca contains only 90mg of caffeine--about as much caffeine as in a cup of coffee