I use Polar Pro filters on my birds. I travel with them by car quite a bit. I've found that the Polar Pro lens cover/locks works very well. I'm not sure how readily available it is in the UK. Here's a link: DJI Phantom 3 Lens Cover/Gimbal Lock
I bought a £3.99 one from Amazon, it has a leg which is meant to fit in the cutout on the top of the camera. I had to shave material off the leg of the cover with a Stanley knife to make it fit easier.
It is a better fit now but still awkward to fit.
The polar Pro is a tight fit on the gimbal platform, which is what you want if it'll be traveling. You should pull the little sticker off the top of the gimbal to make it easier to engage.