What a situation to be in!
Given you have all coped for the past three years with a 5 seater. I would sit down and have a discussion on how often would you realistically need to make use of all 7 seats over a week/month/year. Once you’ve weighed up how often you will carry 7, then bring the cost of running a hybrid compared to an EV and see if its still makes sense or your better on the odd family day out renting a 7 seater.
The 5008 hybrid has a WLTP fuel consumption of 44.6 – 52.5mpg, so regularly driving 200 miles a day will use at least 4 gallons of fuel per day or 16 gallons a month or in money terms a minimum of £100 per month compared to a few quid charging an EV at home.
The Vauxhall Combo xl, Peugeot Rifter and Citreon Berlingo, whils’t van derived are 7 seater EV’s however, do check the mileage.
Given the circumstances, it might be worth speaking to the grants manager and asking for exceptional circumstances based on the lack of suitabal cars and seeing if they are prepared to up the grant or will award the same grant with you paying a larger chunk of the difference.