Hello,
I have a suggestion, enabling non-UK residents to have next day deliveries, same as you brits do.
I suggest either
a) Free Express Delivery with UPS (next-day) or,
b) An option to pay a sum extra for this service.
I’d obviously prefer option a, but even option b would be okay in times when delivery needs to be quicker than usual.
Option a could also have a minimum order of xx(x) euros to be able to use free express.
Non-Express delivery means 2-3 days, typically.
As I said in the chat, not even Amazon offers the option A, at least here in Italy. With Prime, every international shipment takes 2 days (at least). Next day international is a paid option, when possible.
And Amazon is Amazon. So I guess option A is very difficult, unless they set a very high minimum order.
And to be honest I don’t think waiting 2 days is much of an issue, unless in an emergency situation, and certainly I wouldn’t pay for 1 day delivery if there’s a free option.
The problem I had with my Huel order is the dispatch time, that would be great to be reduced, never had less than 2-3 days for dispatch, instead of “same day dispatch” for early morning orders. That would be great, but I understand the Huel warehouse has its limits, and again, not a big issue. 
Where I am in the UK mainland they can’t do next day ,paid or unpaid .