Hi Steve
It depends what your local minimum expected temperature is, and that can vary with altitude, exposure and closeness to the sea, for example. They seem about right to me - Zone 8 expects temperatures down to -9.4 and zone 7 down to -15. Here in Cornwall I reckon we're equivalent to zone 9.
This ought to help -
http://en.wikipedia.org/wiki/Hardiness_zone

I agree Beattie. Here on the Yorkshire coast we are USDA zone 8 but West Yorkshire, although it has warmer summers, has colder winters in general (but this year?) but I reckon you would get away with zone 8 if you use fleece judiciously.

It is easier to look at which plants you can grow and see what zones they fall into. You can try looking it up in the T&M catalogue, and then go to the US edition and see what is listed. The Azores, if you go by temps, is a 11a zone, but I grow zone 9 plants (must be wind chill) without problems.

Wylie, you'd expect to grow plant with lower numbers than the zone equivalent that you live in. Lower numbers expect lower temperatures, the challenge would be to grow plants for zones with higher numbers. I'd expect you to grow plants easily in the Azores (zone 11 did you reckon?) that I can grow here in Cornwall (zone 9).
There is a separate zoning system for expected summer heat. I get the impression it's newer, as it's used less, so far. See
http://www.ahs.org/pdfs/05_heat_map.pdf