Hi Prisca,
You can consult the London/Accommodation section on EasyExpat: <http://www.easyexpat.com/london_en/accommodation_finding-accommodation.htm>.
For 100 pounds per week (about 450 p/m) you should find a lot of offers in zone 2 or 3. Don't forget that the price of the monthly public transport pass is about 75-90 pounds. Look at the Loot and MoveFlat (you will find information on EasyExpat). Ideally you should go to London at the end of August for a few days to visit things directly... it might be easier to be in town and safer to secure the accommodation.
Good luck,

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