OK, so 800m is about 10 minutes walk according to Google maps. The rules are not written down as such but are what I’ve gleaned from various discussions over potential interchanges. Hampstead Heath to Belsize Park is 800m, but to Hampstead is 700m so is more likely to be accepted. What kind of overall journey would you be making to use that interchange?
As to your Warwick Avenue to Preston Road journey, the fare finder does have a couple of options for avoiding zone 1. First is via Kenton and Northwick Park, but if Preston Road is your final destination you can always walk from South Kenton instead if you want. The other one is to change at Willesden Junction and either West Hampstead or Brondesbury/Kilburn. You must touch the pink reader at Willesden Junction.
If you want to make suggestions to TfL then email oysterenquiries @ tfl.gov.uk without the spaces around the @ sign. I would strongly recommend getting someone to proof read and correct your message before sending it because if it isn’t easily readable it will reduce the chances of being understood. In particular make sure station names are spelt correctly (eg Warwick Avenue, not warrick ave) and capitalise each word in a name.