IAG only get a look at the books if Norwegian decide to co-operate . In a full on hostile takeover they don't have to hand over any info and even then:-
"The offeror will invariably conduct a due diligence exercise in relation to the target company before announcing an offer. The extent of the due diligence exercise in the case of a hostile offer will be limited
to reviewing publicly available information, such as the results of searches of public registers and financial analysts’ reports. In the case of a recommended offer, the due diligence exercise may be much more
extensive, but the target company will often seek to limit its extent, either because it does not wish the offeror, who may be a competitor, to obtain confidential information from it, or because it would not
wish the information to be made available to an alternative offeror (see paragraph 3.1 below in relation to Rule 21.3 of the City Code) or because the target company wants to ensure that details of a potential
bid are not leaked to the public.
For all these reasons, as a matter of practice, due diligence in public offers is often limited in comparison with private sales. The target company will, prior to handing over any information, ordinarily insist upon the potential offeror entering into a confidentiality agreement and it would also often seek to include in the confidentiality agreement “standstill” provisions, that is to say provisions restricting for a specified period the ability of the offeror to acquire target company shares without the consent of the board of the target company. There are difficult questions of law as to the "enforceability of standstill arrangement