Is it illegal to spit in someone food in the UK?

In a restaurant or food serving establishment, it would be a breach of hygiene practice. This should (theoretically) get the person who spat in the food fired, or possibly get the restaurant closed (but both of these are unlikely, since the customer is unlikely to know that the food has been spat in, so will not report it to the local health authority. Also, other employees at the restaurant are unlikely to report it, because if the restaurant closed down they would be unemployed). More likely, the person who did spit in the food will probably keep their job.

If the person who spat in to food did so knowing that they carried a contagious disease (whether they work in a restaurant, or are just serving food to others in their home) and intended to cause harm, this may result in a prison sentence.

If however, the person who spits in the food does not carry anything contagious, and is not working in a food serving environment, it is not illegal.