GPS 101
The thing about WAAS or any differential GPS system is that you need to be close to the reference station. The reference station on the ground is at a known, surveyed location. When the ref station receives the GPS signal it calculates its location, and due to errors in the GPS signal (atmosperic disturbances etc) it is unlikely that the computed position will be accurate enough to equal the actual position. The reference station then applies a correction to correct the computed position to the surveyed position, then re-radiates these corrections. Anyone relatively close (ie. so atmospheric disturbances are roughly the same as what the ref station receives) will get a more accurate position. As pointed out, someone in the UK using corrections from the USA will just introduce a bunch of errors into their computed position.
Normal GPS is pretty accurate, most of the time more than 50m which is fine for non-precision stuff.....Better than any NDB anyway.
Cheers
EA