I’m interested in using the Grove - Serial RF Pro in a ground based application.
Are these devices completely licence free and legal in the UK?
Mike
Yes and No 
. They are licence free to use as long as you set it within the exempt frequencies/power levels (around 433mhz band) Depending on your application I suggest you read this OFCOM Licence Exempt Short Range Devices article for full details on the frequencies and power levels…
The other thing to consider is the power level which can also be set according the the application you are using the module for. You can use the table below as a rough guide to convert between dBM and Watts. Although other things such as high gain antennas will effect the final output power.
You can see full details on how to set all the parameters here including some example code for the Grove serial RF Pro module.