The store in Watford Junction is accessable off the Metropolitan Underground line, as well as Silverlink Light Rail. That having been said, you could take the Tube, but to give you an idea of scale:

Central London is Zone 1
LHR is Zone 6
Watford is Zone A (basically, Zone 7)

It's really, as far as London goes, Manchester.

I have been curious, though, as to the differences and have vowed to make the trip to Watford, but if I cribbed my conversations with various Brits that work for me, I'd probably say that unless you're in the market for a skid of Irn-Bru, you're probably better off skipping the trip, unless you happen to already be in the area.

Timothy

P.S. Just checked with a needless consumer in my office who has been to both U.S. and G.B. stores--the general deal is less generic Kirkland stuff, more brand names. However, she didn't see much difference in selection, other than that.