I am also in the UK like you. I currently have some Nest Protect units but I am worried about Google’s moves to effectively kill off the ability to integrate them with other systems.
For the UK the only options I have seen are various smoke/fire sensors. There do not seem to be any combined CO/smoke sensors. Some brands do offer matching separate CO sensors.
There are various Chinese offerings e.g Xiomi, but I find all these Chinese line ups totally confusing and the fact there is no official European sales channel a handicap as it completely removes the confidence in their products being compliant with regulations. There is Netatmo a French company who make a unit which also has HomeKit support but it is battery only. However they claim the battery lasts for 10 years which is as long as the sensor itself would last.
Another option I found is FireAngel. They have an interesting mix of options but it looks like they have an annoying limitation. They do both wired and wireless. They do a range which links multiple units together. They do both Zigbee and Z-Wave modules.
However it seems you cannot have all these options in one. Also it appears they only sell to industry and not endusers and they tend not to respond to contact requests. However here is their site have a look yourself.
It is amazing that after so many years no-one has come close to matching the Nest Protect.
PS. As others mentioned the audio sensors would not work in the UK. A clever workaround someone else suggested is to mount a vibration sensor touching the smoke alarm. When its siren goes off the vibration triggers the sensor which can then report to Smartthings.