Tavistock employs more than 300 advisers who look after 70,000 private clients and £3bn of client investments.
The firm is looking for further acquisitions and will likely use the additional capital and support from Mr Simon to help fund these.
Tavistock and Hamon Group say they are interested in further acquisitions in the wealth and asset management sectors in the UK and Europe.
To raise the capital, Tavistock issued 32.5m of new ordinary shares of 1p each at 2p per share.
The group says Mr Simon, who will own just under 5% of the group, is a “significant new investor.”
The balance of the placing shares have been issued to members of the management team, including 416,700 shares to chairman Oliver Cooke, and 416,650 to chief executive, Brian Raven.
In the UK, Hamon Group owns global emerging market equity fund manager Blackfriars Asset Management which it acquired from BNY Mellon in 2011.
Mr Simon has a background in fund management and wants to develop a “meaningful stake” in UK wealth management, Tavistock said.