I'm a programmer currently working in central London (relocated from Italy about 1month ago). While i don't want to publicly disclose my salary, i can tell you that you can expect to do a bit more than double of what you just mentioned. For a junior-mid position you can expect to get something between 45-55k GBP. For a senior position you are supposed to get something between 55k to 70k. This is assuming you are working with a permanent position, if you work as an external consultant you will probably make 400-600GBP/day.
It also hugely depends on the field where you'll be working, financial institutions are well known to offer better wages (but it's mostly a very stressing environment).
I'm a PHP developer. Strong Testing skills, good javascript. Very proficient with both relation and non-relational dbs. I'm very active with local meetups and try to speak at conferences every now and then BUT only 1.5yrs working experience. Despite i work my ass off, i'm not at all a senior (nor i even sold myself as one).
I've done exactly 7 interviews for related positions in London and the lowest payed was 45k, highest 60k.
I personally know NOONE in central london ever working for less than 45k. Doesn't really matter if you are a C# dev, Java one, PHP, Ruby or pure frontend. Actually, my closest friends use to work in banking and they make A LOT more than what i just mentioned (70-90k, yet they are definitely seniors).
I would love to know where you have seen juniors get £45k. From the developers and companies I know in London this is very much not the case. As other people have mentioned, 18-30K would be more "normal".
It also hugely depends on the field where you'll be working, financial institutions are well known to offer better wages (but it's mostly a very stressing environment).