Take the Piccadilly line from central London straight to Heathrow; it’s affordable, frequent, and stops at all terminals

https://emilianoqxrs791.cavandoragh.org/getting-to-heathrow-from-manchester-rail-coach-and-driving-compared-2

Take the Piccadilly line from central London straight to Heathrow; it’s affordable, frequent, and stops at all terminals, though it can be crowded during peak travel times.