Dude don’t beat yourself up, interview success is quite simply about whether the interviewers click with you in the first ten seconds or so of meeting you. If you don’t click, then you would probably have hated working there anyway. If you do click, the technical questions afterwards are just a formality; you could fail them all and still get the job. Attend as many interviews as you can fit in, there’s no shortage of software jobs in London. and it is impossible to judge the good ones from the bad before you have actually worked there for a few months. The more practiced you get at dealing with interviews, the more confidence you will have, and that’s what will get you the job.