Sweet pots can be grown here, only recently appeared from the seed companies, needs a quite long season (doesnt everything now!!). They arent sold as tubers or seeds but as roots I seem to recall (imagine rubarb or asparagus). Probably only a few varieties available.
The squashes I havent seen over here much (I am from OZ originally) but they are popular in america and oz. They are also known as 'pattypan' squashes. I know vidaverde (add a .co.uk) does sell them, and you can prolly find them among other seed catalogues that do the less than popular stuff.
Hope that helps.
ken