What's so wrong about looking for compensation? If the public didn't have the option of seeking compensation whenever they are harmed through a company's negligence, why would many company's bother their backsides making sure their goods and working practices were safe?
M & S were negligent. They owe a duty of care to their customers and it is completely forseeable that a customer could become ill through eating an out of date sandwich which should never have been on their shelves puting customers at risk.
So, don't be embarrassed at seeking compensation! Seek the advice of a solicitor. M&S are liable in the law of tort/delict for your illness, and breached their duty of care towards you. However they will have a partial defence of contributory negligence in that you didn't check the sell-by date. Despite that, you still have a very strong case.