FTTP can deliver more speed than you currently pay for , but it’s unlikely that you have been put on the wrong speed profile as you comfortably exceed the 80Mb download profile , you therefore are ‘connected’ at at least the speed profile you pay for …this connection is set, it doesn’t vary , it’s not observable by you, by looking at the router stats, or ‘windows’ , and they way the system works you are either working or not, there is no variance like ADSL/VDSL, if the correct profile is set, you get the connection speed you pay for.
As already stated , download and connection speeds are not the same, download is affected by many things that your provider and Openreach have no control over, in your case D/L is pretty close to 100Mb ( which coincidentally happens to be the minimum speed guarantee and the limit of older Ethernet equipment fitted in some PC/laptops …as already stated it’s entirely possible that it’s now your own equipment that is limiting your download speed .
FWIW, Openreach FTTP ( PON) is a system where up to 32 users ‘share’ bandwidth of 2.4Gb download , 1.2Gb upload, if you happen to be connected to a PON that’s got all 32 possible connections in use and many are intensive users , you may see speed dip, but in general it’s more than enough bandwidth, if you feel it’s not enough ( 2.4Gb ) , do a speed test at a quiet time of day , avoiding peak hours (6pm till 90pm or whatever), if no difference then it’s not over-utilisation, again proving the PON is delivering what it’s supposed to and indicating it’s your own gear.
As far as being excused your minimum term commitment because your perceived performance is fractionally less than the guarantee, then that’s unlikely, because it’s more likely that there is nothing wrong and it’s your own equipment that’s now limiting your speed, finally , if you were getting 100Mb ( thereby satisfying the minimum guarantee ) instead of 90Mb , in day to day use there is going to be noticeable difference anyway.
