This has brought its total AUA to £37.9bn as of the end of June.
SEI, which provides outsourced investment technology to firms such as Tilney, Brewin Dolphin and WH Ireland, also saw its platform hit 300,000 end-clients for the first time during the six-month period.
Meanwhile more than 750,000 buy-and-sell trades were made each month.
Martin Steer, commercial director at SEI Wealth Platform, UK Private Banking, said: “The first half of this year has seen some uncertainty creep into the broader market, so it is very pleasing to see another period of strong growth for the platform.
“With regulatory pressure mounting and a client desire for a multi-channel experience, we believe it is becoming a question of when – not if – to outsource non-core functions.
“Our objective is to ensure that our clients are able to navigate through these minefields whilst minimising cost and mitigating risk, and we feel well-positioned to capitalise on opportunities in the second half of 2017 and beyond.”
SEI’s data revealed the 2017 Isa season was much stronger than 2016.
With financial markets bolstering strong investor sentiment and investments continuing at a higher rate over the summer, asset flows onto the platform were up 50 per cent compared with this time last year.