US SIM cards - supplier of these in the UK that work in UK?

US SIM cards - anyone know of a supplier of these in the UK please?

Not for a short visit but for long-term use in both the US & UK.

I need to be able to activate while here in the UK so as to receive calls from the US on it.

In an ideal world, also needs to be a company that allows porting over of an existing US #

(I had a bad experience with AT&T!)

Thank you in advance folks!