Not sure about anywhere else, but in the UK each LLU operator installs their own equipment and own backhaul stuff, so a user connected to say Sky’s LLU (which is really easynet) is connected to Sky owned and controlled equipment at the exchange, which is directly connected to Sky’s backhaul to server/internet backbone. So if you tried to log into a foreign isp, on LLU it would reject you straight away.
At every ADSL enabled exchange a completely separate BT Wholesale DSLAM setup runs, this provides for all isps using BTWholesale. However the backhaul from the BTW equipment to the ISP is channelled for each ISP, with varying throughputs, it is a common myth that all isps using BTW suffer from the same congestion, there can be congestion beyond the exchange that affects one and not others.
These two systems are completely separate. If you are connected to an LLU provider then you can only log onto them, there is no physical connection from them to BTW equipment. If you switch from one LLU provider to another then a cable needs moved at the exchange.
If you are on BTW and switch from one provider to another provider using BTW then it is likely that nothing physical will change at the exchange. However your line is locked (virtually) to one provider, and you need a MAC (migration authorisation code) to allow another ISP to take it over. So even if a foreign isp somehow had access to BTW at a local exchange, if you tried to log into it you would still be rejected. But the foreign isp would need a virtual backhaul from your local exchange to even get this far, and it if had that, then it wouldn’t be a foreign isp.
