"You must admit that it would have rather ruined the whole 'plane getting shot down by bad Clara using a MANPAD' part of the plot if the aircraft had been able to avoid the missile by flying at altitude, blanketing the electromagnetic spectrum with noise and popping flares and chaff as if it were going out of fashion."
To be quite honest, it would have made UNIT out to be somewhat less the incompetent, bumbling fools that they have been so far.
> notClara fires MANPAD at the plane.
>> Cut to plane cockpit, lock-on warning.
>> Close-up of COUNTERMEASURES flashing on the screen, pilot shouting something sharp like "Oh bollocks!" and jerking the yoke.
> The plane banks sharply, flares and chaff dropping like they're going out of style. Cut to credits.
Next week's ep:
> Repeat from the launched MANPAD.
>> Missile detonates in the chaff cloud, plane flies on, unhindered, pilot climbing fast and hard.
>> Cut to notClara is looking somewhat bewildered, and checks to see if she brought more than one. She hasn't.
> Cut to inside the plane, where the Doctor, Oswald, and the restrained Zygon are mashed against the side of the aircraft. The Doctor claws his way to the intercom, demands to know what the bloody hell is going on.
>> Pilot tells the Doctor that they've been fired upon and the countermeasures' auto-deployment saved them.
>> The Doctor asks if he can do that again.
>> Pilot says no, that was all they had.
> Doctor calls Clara's phone, notClara answers after a few moments.
>> Doctor says 'you only brought the one, didn't you?'
>> notClara says she wasn't expecting actual competency from UNIT.
>> Doctor says they can't always be played like clowns, and tells notClara that he's coming for her, and if he doesn't find his Clara, there will be hell to pay.
> Roll opening credits.
But no, apparently UNIT are so stupid and/or critically short on budget that although they can actually negotiate a "President of the World" position that countries which are at one another's throats will both honor in times of alien invasion, they can't afford chaff and flares for said president's ride.