To all your employer has no need to know your reasons for being of sick.
You are required to provide a sick note for the time and that is all. You don't need to inform your work place anything about your surgery, if you are self funding or not is of no difference.
Any information you decide to tell your employer is confidential, they should not ask what you are off for although they probably will (we are all nosy !)
It is entirely up to you how much/little you tell them. Your are entitled to SSP if nothing else, dependent on what your contract of employment says.
The fact that your surgery is elective has no bearing on sick leave, if you are not fit for work you are sick !
Your GP, Surgeon and hospital will back you up, that you are not fit for work, and will provide you with any sick notes you need.
I would say the worst thing you could do is rush going back to work.If you try to rush things it could have a detrimental effect on you return to full fitness, and a return to work.
Hope this is of some help
Ta very glad Dale
OOOhh yeh I have now lost 3 stone 7weeks and only 1 week post opp