There are SO many guided walks to London. Different types suit different people. Based on my own personal experiences, here a few thoughts about the options.
BIG COMPANY, BIG GROUPS
There are the 'just turn up' companies. They make their walks sound intriguing but don't tell you very much about what the walk will cover. They guarantee to take as many as turn up, no pre-booking required. Obviously the main disadvantage of this is ending up at the back of a HUGE crowd, vainly trying to see and hear the Guide. Also, big groups can only stop in a limited number of places, due to the need to accomodate the numbers without blocking thoroughfares. So you tend to stop in fewer locations and stand listening in each place for longer, whether these are the most interesting places to see or not!
INDEPENDENT GUIDES - THE DODGEY
I've seen the big company guides getting very irritated when tourists are misled and poached by someone pretending to be the 'official' tour. There is certainly a danger of ending up with someone who is just conning you and who will take you on (say) a Jack the Ripper tour without having any real knowledge or legitimacy as a guide. Again, this tends to be a 'just turn up' phenomenon - it's easier to end up with the wrong guide in the melee at the favourite starting points.
INDEPENDENT GUIDES - PREBOOKED, SMALL GROUPS
These are a good choice if you are willing to plan ahead. Some require payment in advance, but with some you just have to sign up in advance but you don't have to pay until you arrive on the day. It's worth checking how big the maximum group size is, if any - this can make a huge difference to the type of experience you will have. In an ideal world a group size of 8 is great, although most groups tend to be bigger than this. Upwards of 15 you will probably feel like part of a herd!
DO IT YOURSELF
There are also books, leaflets and downloadable Apps or audio guides if you prefer to do an interesting walk without an actual human guide.
WHAT TO CHOOSE?
As you may have guessed, my own preference is very much for the small pre-booked guided tour. But even within that category there is so much choice it can be a bit baffling. But the good news is that there are lots of fascinating, quirky tours by knowledgable and entertaining guides - whatever your particular interest, there's bound to be a tour to suit.
The going rate seems to be £8-10 for a guided tour (although I have seen some internationally advertised 'walking seminars' for as much as $420 per person!). Most last for 2 hours or more, although within that time the proportion of time spent walking, as opposed to standing and listening, varies considerably.
But what makes a good guide? Well, you have to be able to hear them, so voice projection is a must. And they need to know what they're talking about, so look for either a recognised guiding qualification OR someone who is a published author in the field. Enthusiasm, an entertaining style and friendly approachability are all things I look for.
So, what takes your fancy? Here are just a few Guided Walks I know about personally and would recommend, with links to their respective websites for more details.
Blitzwalkers - The specialists for guided walks on aspects of London in World War II. Really excellent. They offer walks a few times most months, mostly on Sundays, but can also be booked for private groups at other times.
The Lost City of London - Walks focussing on the older bits of the Square Mile: there wasn't much that survived the Great Fire of London in 1666, but if you know where to look, there are Viking, Roman, Anglosaxon, Medieval and Elizabethan features still tucked away here and there. Led by Bob Jones, the author of 'The Lost City of London'.
Footprints London This is an umbrella group for a number of independent guides. I particularly enjoyed their Cheapside walk and (slightly further afield) the Dissenters of Newington Green walk.
Barts Hospital and Smithfield Fridays at 2pm, costing £5. Just turn up (Group size seems to vary from 4 to 20)
Museum of London - on their events page, you can filter to look for the Walks on offer. A mixed bag, and mixed prices, but the Family Mudlarking ones on the Thames foreshore are definitely recommended.
Treasure Trails - my favourite DIY option, and available all over the country, not just in London; particularly good fun with children, but enjoyable for any age really. Bear in mind that you won't get to find out as much interesting historical information en route, but it's a lot of fun, and you definitely see things you wouldn't otherwise see. You can order the trail leaflet by post (£5.99, plus £1.49 postage)- but handier still is the option to download and print out your own copy (£5.99).