I suspect Octopus over time will get less flexible with allowing you to chop and change between Agile and Go. Fixed pricing plans are essentially an insurance policy, and by choosing the fixed plan (assuming it's not because you can't deal with the stress of managing a variable rate plan) you are basically saying "I know better than the energy company". Cost+ is *always* (in the long run) going to be more cost-effective than any hedged fixed cost plan, simply because the energy company has to get the hedge right or they will go bust, so the hedge will always be in their favour.
Even if I didn't have an EV, Agile would still be in my favour, because I run a couple of small NUC servers and a Synology, which consume around 100W. That baseline ends up costing me -on average- less than any fixed cost plan would. Now if on top of that I change my behaviour ever so slightly to do a few things when I know the rate is lowest, then those savings are icing on the cake (I *could* also do the same with Go). However, if I was a *heavy* EV user, and needed my 4h every single night, then there would be plenty of times where I would question charging at 10-15p (Agile when it's high overnight) rather than 5p (Go). So ironically, for some heavy EV users, Agile may not be the ideal plan.
In other words, it's complicated.