I do not think I have ever seen a definition of hire and reward in respect of UK pilots. In respect of aerial work such as banner towing, glider towing, parachute dropping and instructing whether a PPL, BCPL, CPL, ATPL holder, what is hire, what is reward. Hire would imply some formal arrangement with a verbal or written contract, which simply could be an email. Advertising services through social media or contacts could further imply hire. As for reward, if someone who flies part time and costs incurred for travel, hotac, renewels amount to £4000 p/a and income is £3999 p/a, then there is a loss of £1, so can it be argued that there was no financial reward. Again the problem here is, a district judge might consider a long weekend stopover in say Cannes with hotac, food and expenses provided is reward, when compared with the fact the pilot otherwise would have spent a wet weekend in Grimsby.
The problem here is that a definition of hire and reward is very subjective is that a district judge or circuit judge in a court (England and Wales) will work on the balance of probability, meaning they will take a guess on what is hire and reward and likely to take the UK CAA position given in court. The other thing that those operating with the correct licences and ratings, risk losing their assets (home, savings, etc.), or those of their survivors, in full. Again some might think they could protect as a limited company, yet that would imply there was a commercial arrangement in place. Personally, I see it as madness the risk your assets for borderline aerial activities.
The bottom line to this is I am doubtful if there is a definition of hire and reward under the ANO, and there will never be, until the matter becomes case law in the High Court, Appeal Court or Supreme Court.