Just to answer the question posed earlier about whether or not it is illegal to fly overweight - it is in breach of the ANO article 52, which is the article requiring the commander of the aircraft to ensure that the aircraft is correctly loaded, etc etc. If you are in breach of the ANO you are almost certainly in breach of your insurance policy which usually states specifically that it is a condition of the insurers' obligation to indemnify that all regulations including the ANO are complied with.
As an instructor I am always horrified at the number of people who regularly overload their C152s and PA28s. If it was a helicopter you would soon know about it if you overloaded it because you would end up overtorquing and causing significant damage. In aeroplanes, consistent overloading leads to insidious stress to the airframe and quite likely propagation of cracks and fatigue failures. Difficult to measure but I am sure that a metallurgist would come up with some scientific data.
Fortunately I do not usually compromise my student's weight and balance as I only weigh 130lbs. I also have no problem with 18 stone fatties in telling them they will have to learn on a bigger aeroplane - anyway it must be so uncomfortable trying to squeeze in to a C152 with all that bulk, let alone the discomfort for the other person being shoved in to the side of the door as Fatso overspills his seat entitlement. So if you cannot afford to learn to fly in a larger more expensive aeroplane, then I suggest a rigorous diet would be a good idea and you will also save a lot of money from eating less, which could go in to the flying money pot.