I got nice write up on PG1 boxes :
http://www.mgf.ultimatemg.com/group1/info/Gearbox_numbers/index.htm
This source says that U spec is
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this is about 240 Nm (239.9 Nm) i found those info on other sources (forums too) and covers with LR informations. Freelander L-series also got "U" spec PG1, S4EMU. As you say that MG generates 265 Nm PG1 gearbox in "U" spec can handle a lot more than expected.
But i still think that safe limit is about 240 Nm, because of other drive train components
your MG is tuned or stock?