All buses are accessible. And all black cabs have pull-out ramps for wheelchairs.
There are cab ranks dotted about - and you can also go out front of a major hotel and benefit from easy cab hailing there. Sometimes a bell captain will even hail a cab for you.
However, you can call an Uber to anywhere if you have the app, and specify size of car. That might be your most convenient option.
Side note on tube/accessibility: I have limited mobility because of a congenital knee malformation, and often really (REALLY) struggle on the tube. There are - believe it or not - stations where a very long and steep escalator will whisk you in comfort up to, not a level lobby with step-free street access, but instead (surprise!) 2 or 3 short sets of stairs! Which you were not expecting! Argh!
Soooooo… there’s that. :-)
Oh! Also note that the TFL app allows you to filter for step-free journeys. You just need to be aware that if a particular station lift is out of order, that is not updated or reflected on the app.