Hello and welcome to the site. Thank you for your question.You are correct in your understanding that VAT at standard rate would only apply to coffee (hot drinks/food) when you have mixed sales.You could give away free coffee if the customer bought cold food that is not subject to VAT and that free coffee would not attract VAT at the sales value is NIL.You don't have to make coffee free of charge if the customer only wished to buy coffee. You would have to make it clear to customers and on your price list to alleviate any issues with VAT office in the future.If possible you should program your till to account for coffee at normal price and coffee as a give away. What is important is to separate taxable sales with a value from sales with no value.
I hope this is helpful and answers your question.
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It may be benefical in your case to go for voluntary registration if you have expenditure that has VAT to be reclaimed to take advantage of it.
I hope this is helpful.