I have so far not found any HMRC regulations requiring firms to withhold tax or charge personal income tax from investors. However, you may ask them to state, under which HMRC regulations they are charging you tax?
Having said that, it appears to be the industry practice of the Capital investment firms to charge tax from investors as many individuals have been charged by firms.
So TRX isn't doing anything unusual.
If they charge you tax, you may obtain a tax statement from them and then claim it on your self-assessment tax return if it is overpaid.