There are no coincidences, especially in the writing of Steven Moffatt and Mark Gatiss respectively. Re-watching A Study in Scarlet the other night, it all came to me. Doctor Who take place in the same universe and Clara is my proof.
We also know that Clara is openly bi-sexual (Go ahead, try and deny it, faithful reader. I’m waiting) and that it wouldn’t be such a big deal if she ran into a woman she really liked in London around 2010. Perhaps her TARDIS was following the Doctor’s TARDIS and brought Clara there against her intentions as TARDISes are won’t to do.
So Clara has a brief relationship with a certain Harriet Watson. She even meets John at some point but Harriet’s drinking makes anything long term too dicey. And contrary to Sherlock’s assumption, Clara is the one who leaves first and Harriet gives her mobile phone to John.
Okay, I had to alter a detail to fit my theory but oh well, such is the nature of speculation.