NN - yes, Plus.Net and others rent Openreach links from each exchange back to gateway systems in and around London/M25 (various locations connected to LINX).
The ISPs not using Openreach (as much, if at all) would be those competitors with their own equipment at exchanges, such as Orange/EE {previously Wanadoo, Freeserve}, Sky, TalkTalk and Tiscali {now owned by TalkTalk, I believe}. There were also a smaller number of exchanges with BE Unlimited/O2 equipment {but Telefonica's residential customer base recently bought by Sky, presumably with the equipment they are connected to}. Entanet isn't used by many under its own name but have reseller firms like Vivaciti etc. Enta has its own network, as far as I know, but may not be listed by Samknows (think UK is lower importance to Samknows, now, and some data may be stale).
Virgin Media and Sky have used BT/Openreach connections where their own network is not available, but obviously (a) VM doesn't offer TV to the customer and (b) Sky charges much more, as they're paying Openreach for the service.
Easy way in the past to know if someone was on Openreach or other equipment was that most customers of BT/ firms renting Openreach were on "up to 8 Mbps" while other firms were offering "up to 16/ 20/ 24 Mbps" (eg Bulldog, BE unlimited, etc) and of course, if you were in rural Wales, where I was, none of the others was available for years!