We are flying in to Heathrow on 5th June & travelling by bus to Portsmouth by bus. We need a small car for 2 to travel around the UK until 1st July. The website says there are no vehicles available for the dates submitted.
You want to pick up your rental car at Heathrow Airport and return at Portsmouth Port?
I want to collect in Portsmouth & drop off at Gatwick
You should enter "Portsmouth Harbour" and choose that as pick up location. Then tick the box "Return to a different Location" and enter "Gatwick" as drop off location. Just enter Gatwick, not Gatwick North. They are the same place as far as rental cars are concerned. When you don't see cars press: "EXPAND SEARCH" button.
I have send you a typical email quote. Click on the link and you will see the available rental vehicles. From there you can change or adjust dates and times and choose your desired car.