Contaminated packages would seem a weird excuse considering the science doesn't support it, unless I missed somewhere that this UK based mutation is somehow more contageous due to surface contamination which other strains are thought not to be in great effect.
CDC has always been a bit weird with shipping to the UK though, I think they used to mention when checking out returns aren't free and that it's costly (there may even have been an extra charge from them) so to chooses carefully or something like that.
I think a lot of e-tailers are struggling with the constant lockdowns as they need to extend their return periods for both online and physical shops which means they're going to get a lot of useless stock back when sale periods/seasons have passed. I assume the UK/French couriers are currently making a mess of things too. Both countries aren't delivering in a vaguely normal timescale for me, what usually would be 5-7 days normally for me is arriving in 3-4 weeks or getting lost in the post.
Saying that, other retailers seem to still be shipping to the UK and don't think you have cooties 🤷