This only works if you are intending to use another provider that uses Openreach ,
if it’s an Alt Net or Virgin it’s only if you migrate the phone number that your existing BT service will be stopped as a consequence of joining a new provider on Openreach, also if you wait until 25July , then it’s a 30 days notice if quitting completely ( from Openreach ) and 10-14 days if remaining on Openreach , you pay while inside the notice period .
if ( for example ) you need to give BT notice yourself, because the new provider is not on Openreach, and wait until July 25 to quit , you remain connected until around 25 August and pay for that month , however it may be worth doing that in case there is a delay in your new service being connected
