It was very cold during the day on Wednesday so I definitely agree that Sarah was not warmly dressed for the weather - it will really have chilled down by that time of night. Being in London myself and a very regular walker I am slightly surprised by the distance she was planning to walk at that time of the evening. It is quite an empty part of London skirting the Common and with lockdown there are far fewer people around. I regularly walk alone after dark but I wouldn't have done that walk. I guess she knew the area well and was confident - we all have been walking a lot trying to get nearer to our 10,000 steps a day. But it just doesn't look like a really safe journey.
A few thoughts:
1) I also don't think 24 hours is a long time to be reported missing. I live alone and I think it would take longer for my absence to be noticed. I just don't respond all day every day to text messages. And even if she lived with housemates I think its possible for them not to notice straight away if she regularly went and stayed with her partner. I have lived in house shares and no one would immediately notice (though in lockdown life is a bit different). We just didn't monitor each other all the time.
2) I think it very unlikely she will have 'bumped into' a friend. Living in London with so such a large population if I was doing a walk of that distance I'd say it would be pretty unlikely I'd cross paths with anyone I knew. Particularly during lockdown at that time of the evening. Before you have kids you really might not have many friends or acquaintances in your neighbourhood.
3) She was obviously an independent person with a career. I think its perfectly possible that her partner wouldn't think it amiss that she hadn't texted when she got home or in the morning. I'd say quite a few English boyfriends don't check in on their independent girlfriends in that way. Some do but certainly not all. I don't think my ex would have done. I wouldn't have expected him to.
4) Although generally strangers are the least likely cause of crime rather than those that are closest to you, in this case I think its the most likely scenario. I think the most likely option is that she was attacked, assaulted and potentially disposed of in undergrowth or a pond. Less likely, but still and option is abduction. She is petite and would be easy to overpowered and put in a vehicle (with or without her earbuds in). Less likely I think taken into a disused building. Though of course she could have been taken into someone's home. A risky scenario but not impossible.
I'm sorry to say this all so dispassionately. It's very sad and extremely worrying. Just a very ordinary evening for her doing her best to stay connected and see friends after a day working from home during some very tough times in London after extended lockdown. I have no way of knowing about her mindset but I do quite doubt her doing herself harm. I think she would have been having a nice friendly chat with her friend and then chatting with her partner she was likely in reasonable spirits.
Most likely I think she was totally unlucky to have randomly crossed the path of someone who meant her harm. JMHO. So very sad for her.