It's Hypericum, St John's Wort. There are many different species and cultivars, but ttis looks like it could be the groundcover form, Hypericum calycinum, which spreads very vigorously and can easily hit 18-24" inched in height, possibly taller. It tends to be rather prone to rust, so whacking back or mowing at least once a season helps to keep both the disease and plant height in bounds. Very popular in exposed, no maintenance commercial applications.

Not H. calycinum but rather one of the taller ones with permanent woody stems. If you part the foliage of this planting and look inside it you will see it consists of numerous individual bushes grown together rather than a blanket of more evenly dispersed vertical stems arising from a mat of creeping rootstocks.

Probably this planting has been sheared to produce a shorter height than it might have otherwise.

Here is a link that might be useful: hypericum hidcote - Google Image Search