Earlier in the year someone posted quite a bit of detail on it and seems FT are missing a huge benefit for its SIPP customers. The long term value would be pretty darn big as well.
AJ Bell explain here why it doesn’t need paying Do you reclaim or reduce withholding tax on overseas dividends? | AJ Bell Youinvest
And here is a vote to sort it out on FT SIPP: Avoid Withholding Tax on Foreign Stocks - Ideas 
- Freetrade Community