Hello, my name is Ben and it is my pleasure to assist you with your question today. I am not sure why they have told you that an idea cannot be patented as many patents are ideas because the inventors would be at the very first stages of their venture and one of the first step would be to patent the idea before they proceed with manufacturing it, where the idea could quickly become public knowledge.
You will see that many patents are simply drawings with descriptions, before anything physical has even been built. You would usually be required to write down clearly what your invention is, how it works, how it would be used, etc. You would be expected to take out the patent with a patent agent, a list of which you can find on the Chartered Institute of Chartered Attorneys' webpage:
Please let me know if this has answered your query or if you need me to clarify anything else for you in relation to this?