Do what is right for you. You don't need to "make it up" to the company for being sick. Especially as it doesn't even seem like they've been particularly supportive.

Check out what the regulations are, if any, about leaving after sick leave. Check citizen's advice or contact your union. I am not familiar with UK law, but am sure this must be regulated.

I'm sure the company can't ask for money back.  Both parties are following the company rules regarding sick leave, right?  You keep them advised what's going on, Dr's sick notes etc, and they implement the rules regarding sick pay. 

I was thinking of handing in my notice back in Feb due to my diagnosis.  I didn't because I couldn't do it without declaring my hand and I wanted  privacy.  I shared my intent with some people I trust.  One work colleague advised me not to hand in my notice, but to let the company deal with the situation through it's standard processes.  Nurse in the radiology dept said similar - use the sick leave, it's there for a reason.  
A whole tier of my immediate management have now left and it's actually got quite interesting now!
 

Hello Stiltonite7,

                           this is not quite so straightforward as others might suggest,so l appreciate your dilemma.It would be fair to say that by keeping you on fullpay for this period of time goes beyond reasonable,and the lack of contact could signify their good intentions of not wishing to appear to put pressure upon you in your trying circumstance by communicating,Their actions todate are very much suggestive of a caring,concerned,responsible employer which should not be overlooked

There is always a danger in making yourself unemployable by resigning,giving that you now have the issue of health history.There could also be the issue of a future reference which could point to unreasonable attitude to any prospective employer.

l can really understand your feelings after a life challenging illness,making the desire for change of direction a reality. My advice would be to follow this through ,but to return to your employer for a  period of say six months,during which time put all your energies in making yourself a model  employee.This way when the time comes to tender your notice your employer will see will see your leaving as an understandable loss with regret not resentfulness.It will also demonstrate to any future employer of your current fitness ability to work with no issues, and a degree of loyalty and stability.. This time will not be wasted for yourself as it will give you time to assess better your next move,whilst getting back to a full work routine

In short,handing in your notice now will not incur charges,handing in your resignation 6 months on will have personal benefits and rewards.

Its good to read that you are looking to a good future emerging from what has been a difficult period of your life,so l hope your future works out well for you,

                                                                                         David