Looking for small room close to London, from autumn 2016

Japanese friends of mine want to rent a small room close to London for their two children, probably for 6 months - 1 year. They are somewhat sheltered, coming from Japan and being the first time out of the country.

What would be the best place to look for this, or general ideas about price / availability / areas to avoid / areas to prefer? They are 16 and 18, so no kids, but not adults either.

Any further ideas?