Whilst TfL has said that no new diesel buses will be bought after 2020 for single deckers and 2025 for double deckers, some routes may not be suitable for conversion to all-electric power. It hs been said on this forum that the reason why electric buses have not been introduced on route 358, as required in the tender, is because the mileage range of current buses is not sufficient to run this intensive route. There may be problems running some 24 hour routes - when will the buses be recharged? - and many central London routes are 24 hour operations, so I would not expect all central London routes to be converted to electric within 5 years. Hydrogen yes, electric no.
The other problem is that the purchase price of non-diesel buses is substantially higher than for diesel buses, and this will be reflected in higher tender costs. Running costs may be lower, but may not offset the higher purchase price. Given the state of TfL finances, will this be affordable?