My impression was that you can get full LLU, or partial LLU - with ADSL from the LLU provider and voice via Openreach WLR (i.e. SMPF) - but you can't get voice and ADSL from different LLU providers, because there's no way to connect to two different LLU MSANs in the exchange.
That's correct. Lines can only be SMPF or MPF, with SMPF being WLR/LLU.
This complies with Openreach's regulatory obligations as they offer the WLR product. All broadband is 'LLU' we just don't tend to think of BT Wholesale's SMPF as being SMPF.