MCI+2000 is effectively a contiguous-48-states (plus most of Canada) search.

I tested it out: MCI+2000 to LON, one-week stay, all of July. (I figure this whole idea of using other airports is mainly useful when DTW-LHR fares are likely to be very high.)

The query returned results in around 30 seconds, with YYZ being the leading contender with fares in the $700-800 R/T range on Air Transat. AC metal was a little bit more expensive, but not much.

FWIW, ITA appears smart enough to not query *every* tiny airport in the United States and Canada when you do a +2,000 mile search...just the ones that kind of make sense.

Good news for you is that YYZ might actually be a decent option if your actual start point is Detroit.