I have been to London many times and aside from the old underground access has been progressively improving over the years, although the traffic and the crowding is getting much worse. At 7 PM, it can take an hour to get a couple of kilometres on a bus from the city centre. Faster to walk if it’s not raining!
For us wheelies, most underground stations are inaccessible. Quite a few of the overground ones are fine. There are a couple of new lines which are accessible with the whole thing is pretty random.
Your best bet are the buses, almost all of which (including the double-deckers) are wheelchair accessible. Often you’ll have to disrupt a parent with a pram, although wheelchairs take precedence in the accessible spot. If there is another wheelie there you will have to wait as they only let one on at a time. Buses are usually pretty frequent however. Inner-city London traffic being pretty terrible, and about 30% of buildings under redevelopment it can take a very long time to get places sometimes. Oxford Street is especially horrible. The buses are however pretty frequent and the network is vast. You can get pretty much from anywhere to anywhere without the most one bus change.
Black cabs are fast and efficient and the London cabbies are incredible, almost all very friendly and amazingly knowledgeable about this city. Pretty much every black cab is wheelchair accessible to, although it can be pretty tight using a power wheelchair. They all have a flip about ramp from the side door and my power wheelchair at least, which is pretty long, fit sideways across behind the front seat. The two issues for me are that it is hard to get the ramp back into place as my wheels block the spot. The solution is to open the other door and pop my toes out until the driver has closed the ramp. All very interesting. The other problem in my case is that I’m quite tall, 183 cm or 6 foot one and my head squishes up against the roof a little. Not fabulous, but doable if the weather is bad or I’m in a hurry. Otherwise I generally walk or catch a bus.
The ferries are interesting and fun. Only a few are accessible, and not all terminals are accessible. The best ferry company is the Thames Clippers. Their website has comprehensive information on wheelie access. Go to Greenwich! Sadly the ferry to Kew Gardens and further up river to Hampton Court Palace is not accessible (as at 2016 anyway).