What is stopping Pakistan from airing "Perfect Pakistan" or "Pure Pakistan" or whatever else slogan they want to come up with? Nobody. They have to pay money and it will be aired, because it is not controversial
On the other hand, the non-airing of Gaza aid ads was a concious decision by the BBC, because it was expecting THIS to happen:
UN halts Gaza aid after food intercepted - Scotsman.com News
UN halts aid to Gaza after Hamas seizes supplies | The Australian
Hamas calls for Fatah reconciliation, vows to continue fighting Israel - Haaretz - Israel News
Hamas Stealing Food and Medicine Sent By International Donors
And Voila! It happened. The aid coming to Gaza is going straight into the hands of Hamas, who are using to feed their terrorists, and not the people of Gaza.
Honestly, if everything would be the way you described the BBC and Gaza situation, that would mean that the BBC can actually never air out any aid commercial or support for whatever country or place on earth due to fears of the aid "falling into the wrong hands".
Also, let me give you an example about the western media in general and why it has such a bad effect on Pakistans image abroad.
If there was a newsreport about a Pakistani girl who re-invented the wheel or whatever or something positive that would really touch people or leave a good impression behind, it wouldn't be aired or headlined or shown to the big public, it would rather end up as a small piece of news somewhere far behind in the list of actual newsreports, and that's an "IF", most of the time, positive news about Pakistan almost never reaches the west unless Pakistani and Indian leaders kiss/make up with eachother.
Now on the other hand, if we have a newsflash about a man beheading another man in a remote tribal village in Pakistan, or a woman raped, (I hate using the word "rape" so badly, curse the people who commit such crimes), or a bomb blast in Pakistan, a western TV station moreover would pick the negative news over the positive one.
A change in mindsets is needed for alot of people in the west or rather the journalists or the reporters and their news station, instead of the Pakistani government and its people.
They like giving the people what they want to read, and that's "entertainment" in any way or form.
Or the newsstations are simply following the trend that has been set by the U.S. and other major western countries of directly or indirectly torning Pakistans image abroad.
It has a certain effect of "See, this news proves yet again that Pakistan is evil, it will never change".
It's simply not fair, but then again, what is "fair" in our world.