Because of differences between the signals carried by HDMI (digital) and coaxial (analogue) cables, a single cable solution does not exist to connect HDMI to coaxial cable. For transferring between these two signal types you will need an HDMI over coax transmitter. HDMI transmitters, converters and extenders may function differently. This connection requires a transmitter.
Buy an HDMI over coax transmitter. Make sure this device takes HDMI input and outputs over coaxial cable. Prices vary from under £195 to upwards of £650, depending largely on quality and the number of inputs/outputs.
Identify the HDMI source. This device--a Blu-ray Disc player or game console, for example--outputs HDMI signal.
Identify the device--an LCD TV, for example--that receives a coaxial cable input.
Connect the HDMI cable from your output device to the input on the transmitter box.
Connect the coaxial cable to the output on the transmitter box and to the coax input on your TV.
Purchase a transmitter box that meets your needs: These devices vary significantly in functionality as well as price.