Honestly, be well-aware that the value of your salary is going to remarkably decrease if you relocate to London. Mainly because of rent. I used to pay 1.5k to live in Bethnal green (which is ~20/25 min by feet from the silicon roundabout - very likely where you'll end up working). You can save some money commuting but you'll end up paying for transports, so, expect end of the month to spend about the same. If you are alone and will be happy with a single room, it will be 600-800 in a decent shared flat.
I'm 100% confident saying that those 5/10k of salary increase are not enough to justify you to move. 40k in manchester are a lot more than 50k in London.
In terms of salary increase, as a lead PHP dev you can expect to go around 65-70k but that will require time.
Completely different story if you decide to start contracting as in London you can easily ask for 400/500GBP a day.
I hope it helps :)