you can subscribe to this thread and get instant notification of deals other post. Sadly ITA matrix changed a while back and does not allow searching by multiple origin points unless they are in the same country.

re Istanbul, it always worthwhile checking fare rules as it might be possible to route back to LHR the same day, overnight in LHR and continue the next day. I did this in July but admittedly only to AMS. IST on a BA320 or whatever they use is not the best place to spend 4 or 5 hours.