OpenWrt is the way to go on commodity hardware. I use it myself with Zen and have detailed my setup in a blog post that may prove useful. I really have no complaints with OpenWrt (and only the lack of native IPv6 with Zen) for my connection.
I have a broadly similar setup, though I'm running pfSense rather than OpenWrt.
pfSense is likely to have an ARM version soon, which will run on small embedded boxes. pfSense has codel, and hopefully fq_codel will be added in the future. My setup is returning A in both directions on the thinkbroadband buffer bloat test.
Unfortunately there's no RFC4638 support in pfSense yet. I hope to add this at some point if someone else doesn't do it first, but I'm currently too busy to devote any time to FreeBSD / pfSense development. A 1492 byte MTU is not the end of the world, but as my hardware supports jumbo frames, it would be nice to move to 1500 bytes.
So far as hardware goes, Zen usually supply a Technicolor router for self-installs and may supply that for engineer installs. If that is the hardware you end up with, it's said to be quite difficult to use an a bridge.
You can always replace whatever hardware you get with a Huawei HG612 Openreach modem - there's plenty on eBay. Make sure any HG612 is a 3B version with a white power supply. Don't pay extra to have it unlocked as you can easily do that yourself (use the B030SP08 firmware). You shouldn't need to pay much more than £20.
All you need to do with a HG612 is connect an Ethernet device to the LAN 1 port and run PPPoE using the Zen supplied credentials.