Seloco.. The answer is that YOU are the person who should decide whether you are able to comply with the Rules of the Air. It is wholly your responsibility and you must also comply with any additional restrictions in your Special Flight authorisation, which you would have to obtain before engaging in any non-standard aerial activity.
Many, many times a day, light single-engined aircraft are issued with ATC clearance to fly over built up areas whilst en-route from A-B and ATC cannot be expected to know if the pilot or aircraft are suitably "qualified", nor should it be an ATC responsibility to brief every pilot on his responsibilities when a clearance is issued. The system relies on people knowing what they are doing. Thankfully, this system usually works.
In the case of non-standard flights over London (and other big cities no doubt), special authority has to be obtained in advance and various conditions will be laid down in writing to the pilot and he must comply with them. He then contacts the appropriate ATC unit by telephone in advance to see if the flight is possible. If an pilot calls ATC and asks to fly circles over Buckingham Palace for half an hour the controller will determine whether dispensation has been granted before he issues a clearance.