There is no legal definition of vegetarian in the UK.

The Food Standards agency elaborates for us: (my emphases)

There isn't a single legal definition of the word 'vegetarian', either in the UK or in Europe. This means there isn't one set of rules about when a food can be called vegetarian. So what a 'vegetarian' food does or doesn't contain could vary from product to product.

If a food is labelled vegetarian, this usually means that the food doesn't contain any meat or animal-derived additives such as gelatine (a gelling agent derived from animal ligaments, skins, tendons, bones etc.) In the case of cheese, it usually means that animal-derived rennet hasn't been used to make it.

Some vegetarians are stricter about what foods they avoid than others. Remember, if you want to avoid a particular ingredient, you can check the ingredients list on a food's label. Manufacturers aren't required to label foods as 'suitable for vegetarians' because this is a voluntary practice. This means there are many foods without the 'suitable for vegetarians' logo that don't contain meat or animal-derived additives.

Foods approved by the Vegetarian Society can display the Society's 'Seedling Symbol'. To be approved, the food must meet a number of conditions, not just be free of meat and animal-derived additives. For example, foods containing eggs will only be approved if the eggs used are free-range.

Even though there isn't one set of rules about when a food can be called vegetarian, general labelling laws prohibit manufacturers and retailers from describing a food in a misleading way.

Reference: www.eatwell.gov.uk/asksam/foodlabels/asksamlabelterms/#A220002

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