I'm sure it was spelt Anderida last time I saw it written down. It was the name of the original fort at Pevensey in East Sussex, just up the road from me. The name was current during the Roman occupation of Britain but had fallen into disuse by 500AD.
The forest or wood you asked about is north of Pevensey and is called the Ashdown Forest. It was called The Weald Forest by the Venerable Bede in 734AD when it covered the whole of what is now the county of Sussex.
The Pooh connection comes from the fact that the books were set in and around Hartfield in the Ashdown Forest, about 30 miles from Pevensey.
Hope this information is of some help.