Extenders were useful for version three of zwave, but we’re now at version five (zwave plus) and they just aren’t needed anymore. Starting with version 4, which was released a couple of years ago, pretty much any mains powered Z wave device except for smoke sensors will also act as a repeater, which is all you need. Also, since generation 4 all the devices work at full signal strength, there’s no need for a “booster” like you might use with Wi-Fi.
Popular repeaters in the UK are pocket sockets and inwall relays. Also Z wave mains powered wall switches if you have a neutral at the switchbox. But there’s no reason to buy a single purpose extender anymore.
Lightify bulbs use zigbee, so they won’t help with your Z wave issues. Zigbee repeats only for zigbee, Z wave repeats only for Z wave.
There are some Z wave light bulbs you can get if that’s convenient. In Europe there is a zwave plus bulb made by Domitech. They will pair using the generic Z wave dimmer DTH (Zwave doesn’t distinguish between a bulb and a switch.)
There are also zwave RGBW bulbs made by Aeon Labs and Zipabox, but both are considerably more expensive than the Domitech (which is white only, no colors). But all the bulbs cost more than that TKB pocket socket, one reason it’s popular.
Z Wave bulbs will act as repeaters as long as they have power, and have not had the dropped message problem that people have reported with the zigbee versions.