I suspect it'll still go into some of Openreach's network somewhere to enable them to be able to login to the Adva they install.
Full ethernet MTUs and VLANs both work as expected, essentially, because it is an ethernet circuit. You get an ethernet port each end. What you send down one end, appears at the other.
We use full jumbo frames on ours as well as several VLANs from our office to the datacentre.