Hi Christian,
Amazon Echo (Alexa) and Google Nest speakers could possibly be paired with the B2 over Bluetooth and definitely users of this forum have used Amazon Echo speakers in that way.
But...
as I said in my first reply post the B2 will only output wirelessly via Bluetooth or to Sonos speakers over Wi-Fi.
It will not output over DLNA or any other Wi-Fi protocol other than to a Sonos.
Daniel is correct that the B2 can support DLNA/UPnP but only as a DLNA/UPnP media server so you can connect to it as storage (like a NAS).
The B2 does not currently support the other DLNA/UPnP components.
The B2 cannot see or talk to any DLNA renderers on your network. Typically a DLNA renderer is an amp or active speakers.
My Yamaha amp which supports DLNA can see my B2 as a DLNA server but I cannot control the B2 over DLNA.
As Peter said in his post, Sonos works well with the B2 so that or Bluetooth are your best options.
A Sonos speaker may seem relatively expensive but for any other wi-fi solution will need to buy additional equipment as well as your speakers so the cost differential may not be that great.
I did come across these
But not sure of their availability in the UK and whether they would work for you.