The longest TfL contract route in London is the X26 from West Croydon to Heathrow Central, which is 21 miles long.
However if you include other routes running into London from outside, the longest is the 402 at a staggering 29 miles from Bromley North to Tunbridge Wells via Sevenoaks.
Despite recently having it's frequency slightly reduced by the advent of bendy buses, route 38 remains the most frequent route in London with an average frequency of every 3-4 minutes during the peakj periods.
Since the 29 got it's new bendy buses it has been both cut in length and frequency. The 29 now only runs between Wood Green and Trafalgar Square only every 5-6 minutes. Route 29 is surprisingly only a mere 7 miles long!
Route 141 has been extended from Wood Green to Palmers Green to compensate for route 29 north of WN.
The longest night bus route is also 21 miles long, this being the N89 between Trafalgar Square- Erith, closely followed by the N159 Marble Arch- New Addington and N47 Trafalgar Square- St Mary Cray both at 20 miles long.