Pedro. Sorry to learn of your difficulties. The short answer is that I'm afraid your employer could eventually decide to dismiss you on grounds of capability. Whether this happens depends on your employer, its policies and in particular its approach to granting early retirement. There is an ACAS Code of Practice in relation to managing these situations (primarily aimed at employers) so it would be a good idea to get hold of a copy (from CAB).
Suggest also you read the attached link - go down the page to paragraph 8 where it covers "Development of a condition which affects performance in a particular post but which does not, necessarily, affect attendance at work".
This is just one organisation's approach to managing such situations - but it clearly follows the ACAS Code.
http://www.gla.ac.uk/services/humanresources/c apabilityprocedure.htm