This is because you are dealing with clients for whom £50 is the upper bound rate of the developers they deal with.
If you want to increase your rates and have a skillset for it, try doubling your rate and going after clients that are used to paying double your rate.
I have the problem in reverse. I get well paid for my part time remote gig when it's on (say around £80ph - I'm not london based). Local gigs want to pay around £45-50, and given my current situation I'm reluctant to commit to full time at that rate. So I quote higher and they lose interest.
If you want to increase your rates and have a skillset for it, try doubling your rate and going after clients that are used to paying double your rate.