From TfL

Customers travelling on the afternoon of Tuesday 23 June, and on Wednesday 24 and Thursday 25 June are advised to check before they travel and to consider if their journey is necessary, as some rail and Tube services may be disrupted due to the very high temperatures forecasted this week. London Overground and Elizabeth line services are likely to be most affected.

Customers who need to travel are advised to consider travelling at quieter times, to allow more time for their journey, and to carry water with them. London buses are expected to run as normal.
 
We are working to minimise any disruption as much as possible. 

Please check before you travel 
https://tfl.gov.uk/tube-dlr-overground/status/
 or use our travel tools to help you plan your journey.https://tfl.gov.uk/plan-a-journey/?cid=email_FINAL_CON_HOT+WEATHER-journeyPlannerCTA