There are heaters available in various sizes and heat outputs and they run on electricity or may be attached to your central heating for wet heating. They are very good and don't take up any space. You may have more than one, so that each of your favourite working spots keeps your feet warm.
Maybe Perry has had good experiences with under plinth heaters, but I'm afraid I haven't.
Plinth heaters are a little noisy, but that really is not the issue. Electric heaters are expensive to run and need a separate timer so as not to be running continuously. Setting them becomes a chore after a while, and I have often see heaters that are never used.
Those connected to the heating water in principle are a better but have other problems. Because the filter and exchanger are near the floor, they clog up quickly with lint and fluff which is both difficult and tiresome to remove. Heat output can be an issue too. It is so easy to choose a heater with an output that is simply not up to the task in hand, particularly if it keeps getting clogged with dust.
Fair enough, if you have a small kitchen and cannot fit another form of heating, then so be it, but personally, I would make it a last resort.