Whilst hunting for any form of alarm with Homekit support, I only came across two options. The Honeywell Lyric, which seems to be US only. And the DSC Iotega, which their website says is available in the UK: http://www.dsc.com/index.php?n=Products&o=view&id=2628
Does anybody know who can supply/fit this? Or any alternate Homekit options?