Here is the issue:
<date>20080301</date>
That is from the xmltv file. I always treat this date as the date the show was created (for example the date a movie was made), or when a show was initially aired. Further more, as you mentioned, the mxf file contains originalAirdate="2008-03-01". Both can be interpreted the same way: the show was created originally on 2008-03-01. Literally the Microsoft tag that I found closest to this meaning is "WM/MediaOriginalBroadcastDateTime". That is where I put that particular date.
Apparently, MS does not use "WM/MediaOriginalBroadcastDateTime" to hold "original broadcast", but to hold "this broadcast". They must therefore put "2008-03-01" in another tag.
You can do this for me:
Import the BBC News recorded by WMC into MC, and check the MC log file for entries like the following:
0043254: 1924: TV: JRWTVVideoInfo::Open: GetAttributeCount returned 0x0, count=19
0043255: 1924: TV: JRWTVVideoInfo::Open: Attr 0: name len: 12, type 1, data len: 6
0043255: 1924: TV: JRWTVVideoInfo::Open: Name WM/Language String value en
0043255: 1924: TV: JRWTVVideoInfo::Open: Attr 1: name len: 17, type 3, data len: 4
0043255: 1924: TV: JRWTVVideoInfo::Open: Name WM/MediaIsRepeat Boolean value 1
0043255: 1924: TV: JRWTVVideoInfo::Open: Attr 2: name len: 18, type 4, data len: 8
0043255: 1924: TV: JRWTVVideoInfo::Open: Name WM/WMRVEncodeTime QWORD value 133280424000000000
0043255: 1924: TV: JRWTVVideoInfo::Open: Attr 3: name len: 9, type 1, data len: 12
0043255: 1924: TV: JRWTVVideoInfo::Open: Name WM/Genre String value Drama
0043255: 1924: TV: JRWTVVideoInfo::Open: Attr 4: name len: 17, type 1, data len: 32
0043256: 1924: TV: JRWTVVideoInfo::Open: Name WM/WMRVProgramID String value EP00511651.0085
0043256: 1924: TV: JRWTVVideoInfo::Open: Attr 5: name len: 20, type 1, data len: 18
0043256: 1924: TV: JRWTVVideoInfo::Open: Name WM/MediaStationName String value 5-2 COZI
0043256: 1924: TV: JRWTVVideoInfo::Open: Attr 6: name len: 23, type 1, data len: 168
0043256: 1924: TV: JRWTVVideoInfo::Open: Name WM/SubTitleDescription String value The detective suspects Captain Stottlemeyer's girlfriend Linda could be a murderer.
0043256: 1924: TV: JRWTVVideoInfo::Open: Attr 7: name len: 16, type 3, data len: 4
0043256: 1924: TV: JRWTVVideoInfo::Open: Name WM/MediaIsMovie Boolean value 0
etc.
You can post what you find. I hope to see 2008-03-01 somewhere, but it is also possible that they use a different format (other than string format), in which case it will be harder to identify.
Note 1: You need to turn on logging. It is under Help > Logging.
Note 2: If you don't see the logging lines as shown above, you should select the file, in MC's Standard View, right-click, and choose Library Tools > Update Library (from tags).