The only thing that can tell you whether your claim will be acceptable or not is your own university's mitigating / extenuating circumstances policy.
However, your decision to apply later may have an impact on your outcome. For example, at my university, you are usually required to apply for mitigating circumstances before, or within ten business days after, the deadline or exam.
If the application is later than that, you would need additional evidence to show there is a good reason for the late claim.
But universities vary widely in their approach. I go to Imperial College, but the UCL policies are very different.
From my experience, and that of my friends, provided you have sufficient evidence in accordance with the policy, your claim should be accepted and treated sympathetically. It is vital you schedule a meeting with the pastoral team in your department immediately to discuss the claim and what happens next.
It may be possible to "condone" or "compensate" the mark, especially if it is only 0.5%, or alternatively you may be able to re-submit that assignment for full credit.
If you have any further information you can post it here or alternatively my DMs are always open.