How far out do you want to be?
People commute into London from hundreds of nearby towns and cities - Brighton for example is an hour away; Woking is 25 minutes away.
Alternatively London is huge - a lot of the outskirts of London can be technically further by public transport than commuter towns. You could stay in Kingston, or Croydon, or Stratford, and get in easily. These areas are also within the TFL area, so you can travel in using an Oyster or contactless payment cards, rather than needing a train ticket.
(Short answer - look on a map, and then compare it with TFL / train ticket sites)