(I refer to ‘autism’ a lot here because Aspergers Syndrome is now part of the Autism Spectrum Disorder diagnosis. Also, sorry this is so long!)
It's kind of impossible to know how common/rare Aspergers Syndrome is because not everyone gets diagnosed until later in their life, if at all. Since a large majority of the 'symptoms' are social, some people are able to hide them through mimicry and learning the skills that most people are born with. 'Faking' being neurotypical can be necessary a lot of the time, because often your ‘Autistic Response’ to something is the behaviour people call weird and avoid you for.
For example, if I went to a job interview I would have to fake being neurotypical, because eye contact is very important in trying to get a job, but ordinarily, I can’t maintain eye contact without feeling very uncomfortable. At work, I make eye contact with customers because I could lose my job if I don’t seem approachable, but it causes me emotional and physical discomfort, as well as tiring me out. Another example would be stimming, which is considered socially unacceptable. I stim as a sensory regulation method in front of friends and family, but if we have guests or I am around people who are likely to judge me I have to internalise whatever panic/anger/stress I’m feeling and suffer with it until they leave, rather than rub soft textures on my face to soothe it and ground me. In public or with people they don’t necessarily feel safe around, many Aspies don’t show obvious traits at all.
In my experience (as a female Aspie) I appear 'more autistic' in certain situations, such as when I am very tired or have just left a noisy environment, but in everyday life nobody really notices - I've had people flat-out disbelieve me before because I "don't look autistic." The thing with the autistic spectrum is that it isn’t a 0-100% scale, but more like a colour wheel, with different people experiencing different levels of each trait. So, I don’t really speak with much of a monotone and I understand similes, metaphors and sarcasm (potentially because I was an obsessive reader as a child); I still struggle a lot with sensory issues and have issues judging what is and isn’t appropriate to say sometimes though.
Something to consider is that the people you meet behind bars and in train stations may not be autistic because many autistic people find those environments too difficult to work/be in. I find travelling incredibly stressful, and until maybe a year and a half ago (before I built up coping strategies) I would literally cry in shops speaking to cashiers, to the point where I would rarely go out and buy things. Even now, I avoid going out into public places unless it’s necessary.
Social cues are like any other language - you can learn it with enough practice. And even if that doesn’t work, it’s often easier to just parrot what you know is acceptable to protect yourself.
In answer to your actual question, I’m aromantic asexual so I’m not pursuing a relationship, although I’m fairly certain I’ve never been considered attractive. That could definitely be to do with my personality and reputation, as well as my willingness to not hide my Aspie-ness from people (because it’s nothing to be ashamed of, and it helps you find real friends if you be yourself up front). Don’t know how relationships are for Aspie guys, though, since I don’t really know any personally.
(Also, I know the question about central London wasn't adressed to me, but I absolutely hate London (especially rush hour Tubes omg). I've only been a couple of times but I cope by wearing ear defenders to dull down noise, paying particular attention to my sensory needs (so constantly adding/taking off layers to keep my temperature comfortable, wearing lip balm because I can't stand the feeling of my lips being dry, that kind of thing), eating and drinking a lot so I don't get tired and my sugar levels don't drop and taking frequent breaks in the quietest coffee shops I find - because if I'm not feeling my best then that's when I have shutdowns and meltdowns and panic attacks. If this didn't make much sense, Googling 'Spoon Theory' might help, it's a fairly decent description of living with conditions such as autism and chronic illnesses).