Is an employer legally required to pay for subsistence expenses incurred whilst an employee is out of the office, e.g. a coffee and sandwich if the business trip spans lunch
Maybe you did. But that wasn't the question. The employer has no legal liability to do this, unless it is specicially in the contract.
Best not to confuse the issue by explaining your own situation.
Normally if I was working out of the area my employer would pay for Bed, Breakfast and an Evening meal.
Oddly they paid for soft drinks with the meal and for 2 phone calls a day in the days before the mobile phone.