Where is BT area code BT0208 890?

There is no such area code as 0208 in the UK. The area code is (020) for all of London. A number starting (020) 8890 xxxx will normally be in or around Feltham, Middx. However, remember that it is sometimes possible for people to take their phone numbers with them when they move out of the area - so it's not guaranteed! The number should be written as (020) xxxx xxxx or as +44 20 xxxx xxxx.