That is how I make most of my purchases in the US, as I am in South Africa, and the South African postal service can't be trusted regardless of whether the seller in the US is willing to ship here or not.
It works well for me, but it's only really cost-effective when buying several items and consolidating them into a single parcel before reshipping. However that's because the only good options for sending to South Africa are courier options (DHL, Fedex) which are more expensive.
For sending to the UK it should be cheaper and more straightforward, as one could reship using USPS.
Not sure if this information is helpful at all.