I might be right, but I think the restriction is on the seller/exporter. To sell beyond 4x scope mag they need a licence to do so.
Ideally you need to look for a company with the right export licence, but even then I'd be cautious. If a private individual sends you something then it needs to be marked as a gift. Some other paper work may well be necessary in the US to make sure it can pass through customs, but even then it might get delayed or even rejected.
There is also the VAT look out too which you will probably have to pay regardless.
https://www.gov.uk/goods-sent-from-abroad/gifts
It isn't clear but I'd stick to companies with an export licence.