There is a separate ground based DGPS system active in the UK, transmitting at around 300kHz. Its designed for shipping but I don't see why aircraft wouldn't be able to pick it up (since we can pick up NDBs on similar frequencies). It probably would need an additional aerial though.
Satellite Navigation
However the Garmin website only mentions SBAS / WAAS for Aera 500. Perhaps it is picking up the EGNOS signal but doesn't know what to call it - so it is defaulting to calling it "DGPS" instead? The WAAS satellite signal isn't likely to reach Europe.
My understanding is that all of the satellite based augmentation systems (SBAS) WAAS, EGNOS, MSAS, GAGAN (when it goes live) are all compatible with each other. So if you see DGPS on your Aera while in Europe it will be using EGNOS.