The fund manager, whose firm is Friends Life’s eighth largest shareholder, spoke out for the first time since the deal was announced on 21 November.
Buxton (pictured) told The Telegraph: “This is a case of two plus two equals a lot more than four.”
The combined company, which will be called Aviva, will serve 16m customers – a quarter of UK households.
Jupiter Asset Management senior fund manager Alastair Gunn, also a Friends Life investor, has been upping his stake in the firm in recent weeks and also plans to give his approval.
He said the deal would give “exposure to a stronger long-term growth story” and the combined company would be run “by an impressive management team”.
Other industry figures previously said they feared a merged insurer will mean worse customer service and less competition.
The comments came as the two companies prepare to send out circulars to investors laying out the full terms of the deal and giving them 21 days to cast their vote.
Aviva employs 12,000 people and Friends Life 3,500. The firms have hailed the creation of a “British champion” and claim they can cut £225m of costs by 2017 by getting together.
