yes - you say maps are bad too,and streaming the most basic YouTube content can be a struggle.
But do three throttle utube content ? below is a 2016 comment
We use video optimisation to change the way video content is streamed to your device to make it play better. We use it to improve your streaming experience if our network is busy where you are.
youtube uses adaptive dash - so perhaps three have additional controls to drop it back to lower resolution and poorer framerates
is there any data on total bandwidth capacity each provider has for London ... is three immediately down. ?