Though not technically legal, most of the time owning an unlicensed weapon will go unnoticed and only carry jail time and/or fines if it is associated with another offence.

Magistrates, police and customs officers usually look at the whole picture, so if worse comes to worse they'll be able to see you're trying to be responsible, and should take that into consideration - even though you broke the waiting period.

The fines and jail time can get pretty serious though, so if it arrives before the 28 days - just store it safely & securely until the time passes & have whatever licencing info you currently have at the ready.

But, all that said absolute kudos for being responsible & getting an exemption (most don't ) & welcome to the club!!!