Humpty
I presume you are refering to fusing vision. In order to eliminate any double vision, the brain has to process images from the two eyes, and make 'the complete picture'; this process is known as fusion. Sometimes, (especially if a squint is present), it can go wrong, leading either to double vision, or disregarded imput from one eye.
For example, if a child has an undiagnosed squint, cataract, etc. and if NOT treated before the age of about 7, the brain has problems fusing the vision, so instead 'switches off' the image from the bad eye. If this happens, binocular vision does not develop properly.
You will be tested for fusion and binocular vision at your medical, you will probably also be tested for accomodation (focusing up close). If you are found to lack bincular vision, this means that you MAY also have no depth perception either.
Hopefully, the problem you have sounds very slight, so none of the above will apply to you.
If I were you, I would go back to your optometrist, and ask for a detailed explanation, and find out what you do, or do not, have.
Alternatively, go and get just the vision part of the medical done at Gatwick
Good luck
FD