This is the best answer. Very easy to fit, and fairly cheap (sub £1k). It's the cheapest form of electric heating you can get: roughly a third of the cost of running a panel heater. You can also control the schedule from your phone on many of the wifi units, so can have it toasty-warm just in time for you to arrive. Take a look at the "mini splits" discussions on Pistonheads and other fora.If you think it might get hot in the summer, get an a/c unit. Most of them will work as air source heat pumps for heating in the winter.
Work well but must insulate the under floor well first.Have you used any? Been looking at these underfloor electric mats,