At present there is a Europeanwide ban on the import of daylilies until further notice due to the fact that they can act as hosts to xiyella (fastidiosa?) Virus which if allowed to enter agricultural crops in Europe would be a disaster, both environmentally and economically I understand but I don't know the full details.
There are a couple of growers in the US that have been inspected and declared disease free and can send to the UK but it is very expensive to order as they have a minimum order value and phyto certificate costs on top plus import charges and taxes and the plants might STILL be destroyed by Customs after all that.
Also I am assuming that to retain their clear certificate those nurseries can't import from other parts of the US without paying for reinspection so they are not going to have be latest varieties, although that is just speculation and surmise on my part so please don't quote me on that!
I think that the ban has been in place for around three years now, maybe someone else can clarify that.
Of course we can send plants WITHIN Europe but only ones that were already in Europe before the ban came into effect and we have nowhere near the same choice and number of varieties as are available in the US, especially in Britain.