In the UK, it is not mandatory to label your products as either vegetarian or vegan. However, many products - if vegetarian or vegan - will voluntarily state if their product is veggie or vegan friendly, some even go as far to say 'Not suitable for vegetarians' which makes life a lot easier.
If a product has a green 'V' this means that the product is vegetarian. We do have an official symbol for either vegetarian or vegan food, this is endorsed by 'The Vegetarian Society' - basically The Vegetarian Society will check how the product is made and with what and will then allow you to use their seal on your product. Sometimes a product will just say 'Suitable for Vegetarians' or 'Suitable for Vegans'. Many restaurants will use symbols on their menu to identify the veggie and vegan food, generally a 'V' (normally green but not always) is used for veggie, and another symbol chosen by them will be used for any vegan food.
All of this makes shopping so much easier, though I still come across products that do not have any clear indication as to whether it is veggie or not, resulting in myself checking the list of ingredients. If I don't see any ingredient that I know is not veggie, then I will do a quick google search as sometimes companies will say that a product is veggie/vegan on their site, though if I am unable to find this information then I will not use the product myself.
We do have some products that are instantly regonised as vegetarian such as Quorn or Linday McCartney's, both of which also have a vegan range which they clearly display on their product if it is vegan.
Also, supermarkets generally have a Vegetarian section in the frozen isles, within this section you can also find vegan products also. This section generally stocks the supermarkets own veggie/vegan range or ranges that only make veggie/vegan food such as Quorn and Linday McCartney's products. You won't find your frozen vegetables here.