It's actually much more nuanced, legally, than just to say that you are allowed a copy.
You are allowed to create a digital copy of content that you hold. So if you create the back-up of your disc, that is fine. It is illegal to download a copy of a film - that isn't a back up of your disc, even if it's exactly the same. Uploading a film is also illegal, as it is a crime to distribute copyrighted content.
UK law is a bit grey here, because nobody (as far as I can see) has ever taken this to trial and argued that they did have a physical copy, but downloaded another. They usually settle before that point. That may be because court is expensive, or because their legal counsel advised that it isn't a strong argument.
It's simpler in the US, where the law explicitly states that downloading and uploading motion pictures or any other copyrighted material is illegal, and that there is no special provisions for situations where you already own a copy.
The three countries where this isn't illegal, and that have no international agreements, are San Marino, Turkmenistan and Eritrea.
It's unlikely that you'd get caught or tried for this unless your ISP reported you, but it is against the law. You could always test the argument in a UK court so that we all know where we stand!