Be aware of the Oak Processionary Caterpillar and harm it can cause (it is an invasive species and not native). It can cause rashes, throat and eye irritations, breathing difficulties to humans and animals
Have a look at the Forestry Commission's information page to help with identification and DO report if you see any on your trees- they are trying to limit the spread.
Currently sightings: Hertfordshire, Essex, Surrey, West Berkshire, South Bucks, as well has areas of London (some of these sightings are historical but they may still be living there- do check)
Forestry Commission Oak Processional Moth Site:
Video for identification