Not sure if they ship to the UK, but take a look at this.
I saw that they also make one for the Mavic Pro which would actually fit the Air. Asking for one to fit all three is... um... asking a lot.
That screams custom built. If you build it, PLEASE post pictures.
Good day. I understand that you are looking for a case that can fit both the DJI Goggles and the Mavic Air. I will be posting a link. This link will show DJI Goggles/Mavic Sling Bag. Please try to check it out.