There's a series of national rail strike involving RMT members on July 27th, August 18th and 20th. If it's similar to the last strike week, London Overground will operate a skeleton service until around 6pm and Southern won't bother at all. Southeastern didn't run any services on the Hayes line, but trains were operating from Lewisham.
Aslef members will strike on Saturday July 30th. In our area that affects London Overground and Southeastern but Southern are NOT part of the strike action which fingers crossed should mean the normal 2 trains per hour service to London Bridge will operate as normal.