Because the survey note says ‘served by 2.5inch duct’ , there is a pretty good chance your installation will be relatively easy as your address presumably is a relatively modern property ( probably build after 1990 ) and although you don’t state it , it’s probably going to be a single stage install, so the date you are given is the date installation should take place ( a two stage install, which have a longer lead time , there is an engineer visit before the contractual date to ensure that on the install date the install any issues identified by the survey have been addressed )
In your case , short of an unforeseen blocked duct your install should be OK , however as suggested, if having a continuous service and no possibility of period without service from either ISP , getting the BT service in before cancelling VM is a good idea , it may only be a month of paying for both services versus potentially having no service for a period.
