Volume - Button Stuck in on position on Iphone SE, Where to repair and what to do in the meantime?

Hello,

A few days ago (the weekend) I started having problems with the volume down button on my iphone SE, clearly at the hardware level. Firstly, it only works in fully pressed position (clicking it reduces the volume to 0, I can no longer put it down one notch), and simultaneously the "internal - button" has slipped out from the external button and activates at sometimes unpredictable moments, including by light pressure at certain parts of the iphone's external shell, and especially accidentally through the volume+ button. (The - button seems to have migrated towards the external + button, which results in frenetic pressing of the volume+ button to stabilise sound at the desired level and to deactivate the volume - button.) I was using the control panel instead to control sound level but accidental volume - button clicks, often through merely brushing against one volume button or the other, or sometimes through simply touching other parts of the iphone, were frequent and intempestive .

Today the "- " button finally stuck itself in an on position, (resulting in a permanent "volume mute" volume box in the middle of the screen). Thankfully this button's continued activity doesn't interfere with normal iphone use and I was able to back the iphone up and am using it as normal ( though mute and so not as a phone obviously).

I'm obviously going to have to get it repaired, but where would be the best place to go to? An Apple store, or an approved repair shop (assuming non-approved repair shop are a less good idea?). (I currently live in London). My iphone is nearly two years old and thus I doubt the warranty still holds. How much would this probably cost me?

In the meantime, is the constant activity of this button liable to damage my phone? Should I turn it off or would that make it worse?

Thanks in Advance for your help!