While not a flyertalk guru - hope I can help! I have flown MAN-ORD/IAD a couple of times on W (cheapest) fares and have got full credit for them (this was earlier this year). Wingnut's excellent guide in the forum sticky confirms that discounted economy Transatlantic gets full miles though DC do not make this clear - indeed the mileage calculator on the website is misleading.

If you are near LHR however it is probably much less tiring to go direct on UA and avoid the connection in MAN. You can then take advantage of the UA double miles and avoid the risk of bad weather leading to the MAN-LHR shuttles getting backed up which is not unheard of at this time of year. You can even use the Sunday daytime return from IAD to avoid having to use any holiday allowance and still stay a Saturday night. It does however make for a particularly uninteresting weekend!