I think if you're coming at this from having a small 2ch Hi-Fi such as a mini system, but you find most of your music is now streamed, then one of the smart speakers makes sense.
In this scenario, you'd no longer be playing physical media. The web could provide virtual radio and access to online libraries. Home Wi-Fi provides NAS/network music access. Voice and smart phone app provides control. However, you've been using an AV system, and that provides a broader range of physical connections (Opt, Coax, analogue, maybe HDMI). It also takes care of decoding digital audio. You want to reduce the physical size of the system, which is understandable; but can you live with losing the connections and decoding in the process? Coming at this from having an AV system, I'd find the smart speaker option just far too limited.
If it was me, I'd look at one of the AV sound bars such as a Yamaha YSP2700. It keeps the AV functionality, does music pretty well, and - with the addition of a £30 Google Home Mini or Amazon Echo Dot you can have all the voice control goodies too. But there's a problem; it's teh price. The 2700 is just short of £700 which is a fair chunk of change over and above your £400-ish budget even if it's a far more sophisticated piece of gear. There is an alternative though. The YSP2500.
This is an earlier model but still does a lot of the functionality.
LINK. It even copes with 4K @ 60Hz. There are limited numbers of factory refurbed units available for a shade over £400. Add a Google Home Mini with its Bluetooth connection and you'll be up and running for around £450.
