Takeovers are not quite as easy as you think....
Limits on foreign ownership may be set out the Memorandum and Articles of Association of the company (and yes, these can be changed), the CAA may also have something to say on the matters, there could be competition issues on certain routes which could require disposal of slots. Its difficult to buy 49% and then join forces with other shareholders because your agenda will be very different...the banks, pension funds will want growth or dividends but won't be botherd by the operation of the company, Emirates on the other had would want to be involved in the day to day activites..not what plc shareholders normally do.
And theres plenty of other factors:
Takeovers never seem to work very well
Emirates and BA are totally different...what would Emirates actually gain (and don't say routes, revenue and profit because they would be paying for them)...apart from a big pension liability, a unionised workforce (I know we all have differing views on the organisation of labour but if you are buying a company you won't be a leading light in the union movement) and a lot of other large financial commitments....
Emirates have got plenty more opportunities in the UAE / MENA areas....they're interest to look at the mistakes the legacy airlines make and ensure they don't make them....
So thats settled, Emirates is noting taking over BA......but I did hear of a Mr O'leary looking at the share price this morning