It would be difficult to find a cordless mower that can handle 750m² on a single charge unless you are cutting very fine amounts of grass with very little load on the motor.

I would suggest the LawnMaster 48V 41cm, as it comes with spare batteries. Each charge will give you around 400m²+, so with the spares, you can comfortably cover both gardens. You can find it here: https://tidd.ly/3UotZ6z

If you do decide to purchase, you can use my 10% discount code EASY10, which brings the mower down to within your £350 budget!

Kind regards, Mark