I’m installing 18mm solid wood flooring – can I still use UFH below this? 

The responsiveness of any UFH system is dependent upon the thickness and density of any surface laid above the heat source. For example:

UFH in a conservatory with a 6mm laminate floor finish will heat the room space quicker than the same room with a 15mm ceramic tile floor.

So… it is perfectly acceptable to put UFH in a room with 18mm solid wood - and although the responsiveness of the system will be compromised to some degree when compared to a thinner flooring, will still heat up quicker than any other electric wire or water based UFH system.