I have AJ Bell who manage TD waterhouse
This is their reply
Please be advised that you can invest in the USA and Canada however the stock must settle via CREST in order to be held in our SIPP.
You can check CREST settlement via
www.euroclear.co.uk or by calling the Sippdeal Dealing Services Team on 0845 373 3479.
It may be possible to obtain a live quote in GBP between 14:30 and 16:30. However, we are reliant on an online price being available to ourselves.
Please note that all trades are in GBP and limit orders are not permitted.
If you have any questions please feel free to contact us.
Yours sincerely
Roger Mills
Dealing Services Team