In the short term, a lot depends upon what it says in your contract. This subject may be covered under sickness or sick pay. (As an example, mine is something like 6 months at full pay, a further 6 at reduced pay, then finished)
More generally, an employer isn't obliged to keep you on if you are long-term sick (and most people would say 6 months+ is long-term). You can't be forced to resign, no, but they can just sack you.
You don't mention sick pay; are they making up your SSP? You can't expect them to go on doing this indefinitely.
Understand their point of view: the company also has a responsibility to its owners, its customers, the other staff. They have no idea of when you'll be back, so can't make any plans.