It could be picking up MSF 60kHz (ex Rugby, now Anthorn), but is isn't designed to work with the MSF time code (at least the manual doesn't even mention MSF).
The WWVB time code is totally different from MSF. It is absolutely impossible that a receiver taking MSF for WWVB would display anything sensible (it probably wouldn't even find the minute marker).
Under good conditions, WWVB is recieved even in Hawaii (see quote below).
But after all the discussions about poor WWVB reception I can't imagine it receives WWVB in London. Groundwave certainly not, but possible the skywave (under special atmospheric conditions).
So either it really receives WWVB (wow!), or it has an undocumented MSF decoding feature. Or a neighbour with an LF tranmitter is playing tricks on you :wink:
According to the manual, after a reset, the Mega 1000 shows WWVB adjusted to pacific time (WWVB broadcasts UTC and the watch has to correct for timezones and DST). That makes it a bit mysterious, unless you ajusted it to local time while it was on WWVB. Then it should show the correct time even when on WWVB. (Boston is on UTC -5 AFAIK)
During the nighttime hours, the WWVB signal is strong enough to synchronize clocks in the 48 states of the CONUS, in parts of Alaska and Hawaii, in all of Mexico, in most of the populated areas of Canada, and in some regions of Central and South America. (For coverage maps and signal strength information recorded at various sites, see
http://tf.nist.gov/stations/wwvb.htm.)