The British government may have offered help, and indeed might be supplying it to the multi-national investigation. But as no passengers were British, the plane was not British registered, had not taken off from Britian, and was not in British airspace, we have no formal locus and are unlikely to be able to offer anything that is not already being done by the US ("too many cooks" etc). RR is involved.
And I suppose how much credence to put on the Mail report depends on whether you believe RR/Boeing and the official investigation who put on the record yesterday that there was nothing from the aircraft since the last recorded position, or a newspaper quoting a source which contracts this.
I find it hard to believe that two international companies and all the officials involved in the actual investigation would permit the Malaysian minister to lie deliberately. And if the information was wrong (incomplete/premature), then I would expect a formal retraction on the record, not a choice to leak via a newspaper (and wouldn't a US official speak more readily to a US paper, not a Uk one?)