Agree - spend the money on the ticket - then plan to use the MR + UR points at another time (look at both programs and see which airline partners they have in common).

United fly direct IAD-MAN - it's a 757, a bit cramped in economy, but it is a direct flight which is a big plus. Dec 26 - Jan 2 is $1102 including tax.

Also if you fly into London and were planning on taking the train (say 1hr 15m by underground from airport to EUSTON station, then normally 2 hours by train) the main line between London and Manchester will be having engineering works during the Christmas/New Year week - and trains will be running "considerably slower" - so you really are better off flying direct into Manchester.