These can be innocent or sight threatening and will need to be assessed under the local NHS paid for MECS scheme. If you notice glare as well as a red eye that is a definite pointer for your eyes to be assess very soon. Some clients with red eyes have minor bleeds due to taking blood thinners or high blood pressure.
this page from medical news today provides a useful guide to various red eye conditions.
This is an image of a red eye, which is a Pterygium:
A Pterygium is a skin grown accross the front of the eye. Some people confuse a cataract with a pterygium, however a cataract occurs when the lens in the eye goes cloudy, the pupil will look like the image below: