+1 on Looigi's comments on the 500 and GPS in general. Clouds don't significantly affect GPS reception, precipitation may, tree foliage & tall buildings can (signal absorption, multipath, blocking satellites low on horizon). Biggest practical issue for me are slightly longer time to GPS-sync if my 500 is sitting in some parts of my home before a ride, and errors in GPS position at the entries & exits of Strava segments - Strava may decide you didn't ride the segment you just rode. Other than momentary frustration comparing how I did on a few climbs for a particular century ride, this has not been a big problem, is generic to any GPS-based system, and not a problem if you don't care about Strava.

Garmin markets the 500 at roadies and the 800 for people wanting decent on-bike mapping (randos, touring, mtb, everyone else).

The 500 works well stand-alone (GPS for speed & distance) and with *any* ANT+ speed/cadence sensor (Garmin, others). Also any ANT+ power & HRM sensors. No Bluetooth support. If using the speed/cadence sensor, you can use GPS to calibrate, enter a pre-calculated tire circumfrance, or enter a measured roll-out distance. GPS can be disabled for indoor trainer use with the speed sensor. Rudimentry mapping (follow trail of "bread crumbs", warn when turn coming, when off track (subject to GPS meandering) and direction back to track works well enough. The 800 series has a larger display and real mapping (what you'd expect from GPS navigation). I am quite happy with my 500. Data transfer is via USB cable, and most cycling software and internet cycling sites either support direct data transfer from the 500, or manual upload & convert the raw .fit files. In particular, Garmin's own site (of course), Strava, TrainingPeaks, Ridewithgps, the mapmy<ride|fitness|run> folks (lamest interface, however), GoldenCheetah, etc.

Garmin has announced the replacement 510 and 810. 510 is larger, some on screen menus, better interface for social media/on line use. At this point not worth the $ to me to upgrade, although this should reduce price of 500 & 800 models on close-out. Garmin has a marketing video showing off the networking/social media aspects. dcrainmaker has a detailed review of the 510 & 810 on his blog. Since you're in the UK, watch probikekit.com for deals on Edge 500, 800.