Is meal allowance for the guide a legal requirement?

No, but the employer has to make sure that you receive the legal rest breaks as per the legal requirements by the Department of Labour. If you are a driver guide then logbook regulations apply as well. See here for details and check "Members only" section (Login required) for guidelines to job assignments.

It is common practice though that as the guide you receive the meals that are included in the itineraries (i.e. that are included for the customers) or a meal allowance based on half board (breakfast and dinner) whether you join the customers for meals or not. Please check your guide contract with the employer for meal allowance.