I would be honest with them if you want my opinion as if they find out you are hiding something it could affect your job offer. They will only be supportive about it. Look at this the positive way in that you are doing something about your weight and following a safe plan so there is nothing negative that they could say.
Honestly don't worry honey. I've worked in the NHS for years and for all the occupational health checks I've had done in the past I don't ever remember getting my urine tested anyway!
Good luck and please don't worry- they will probably pat you on the back!