I guess your inverter is not a hybrid one then? Mine isn't so when I decided to add a battery the installer fitted an ac coupled one (details in sig).
Someone else will probably correct me but I allow a notional 7p/kWh and 80% charge discharge efficiency for the cost of storing electricity in the battery. I am still on a 50% deemed export on my FIT as Octopus have not managed to remove me from the FIT deemed export and provide me with an export MPAN for my smart meter. It is largely moot because I do not have an EV so am not entitled to join the Intelligent Octopus and the Octopus outgoing (yet). I am actually quite happy using only my low carbon electricity 8 months of the year.
In your case you can and the 7.5p/kWh for 6?? hours will benefit your driving costs greatly. I am not sure how one would match these variable 6 hours of cheap rate to firing up your heat pump but the thermal inertia of an underfloor system will still benefit you during the day.
My daughter has an EV and has the 9p/kWh rate for 4 fixed hours just after midnight but this incurs a higher rate of 31p/kWh outside of the off peak 4 hours (a 2p/kWh penalty over a standard tariff) so I remotely set her 10kWh home battery to charge off peak sufficiently for it to carry here through to the next off peak period, until the sun starts shining.
Worse case in the absence of solar PV in the winter months I calculate:
10kWh charged at 9p/kWh plus battery cost at 7p/kWh divided by 80% cycle efficiency =10*(9+7)/0.8=200p
So her electricity cost is 20p/kWh, about a 9p/kWh overall saving compared with a standard tariff and of course during the summer months she gets all her electricity at no costs above the capital expenditure.
also moving over to outgoing octopus but that seems to take up to 14 days despite already being on Flux. That will make for an interesting bill next month.