SIPP if moving overseas

xylophone

https://www.fca.org.uk/firms/authorised-recognised-funds

gm0

You are seeking precision and clarity where none may in fact exist which directly links to the provider decision.

They may well prefer to imply it is not allowed.  Even if what they mean is "we don't offer that". And that was a deliberate choice

If the "that" already does or easily could cause us a load of extra tax reporting work for a very few accounts.
So we just - don't.  You'll have no basis to force them to let you buy anything anyway. They do it or they don't. 
They can likely bin you as a customer any time. via T&C.

Businesses just don't want another FATCA debacle for a handful of not very profitable cross border customers - if cross border is not their primary thing.  FATCA if you don't know the example was an international customer tax reporting exercise of blatant extraterritorial overreach by the USA in respect of its citizens or anyone else tax resident there.  Awkward and a pain in the backside to do FATCA reporting ongoing for a handful of customers if you don't generally do business in or with the US - somebody offering services to all comers in a country elsewhere.  Anyone who already had US taxpayers with accounts anywhere - had to quickly unfriend them or keep them and set up to send Uncle Sam the FATCA returns. 
Penalities for not playing along in international subsidiaries in terms of US market access for related groups were used as a stick on the international majors.

In a world like that - it's hardly surprising that risk and product teams will want business policies and edges on product design that deter complex international edge cases from building up. 

They don't make enough money to be worth the hassle.   So we don't do that.  Tell them whatever.  Get the intern to write the web pages.