Ministry of Defence files on UFO sightings released

It depends. If all they have is unreliable, inconsistent eyewitness testimony, then it's acceptable to ignore. If they have physical samples they've not tested or electromagnetic readings they've not had analysed, then it's an incompetent blind spot.

Personally, I would think unknown objects in the air over the U.K. would be a matter worth investigating regardless of what they could be.

What if they were some crazy Soviet nuclear attack airplane or something? Or some sort of weird spy plane? Or, well, aliens.

I don't see what is odd hypothetical about having one hard technology but not another hard technology.

Intersteller travel is only hard from a physics point of view if you want to go from X to Y and then back to X, with not too much time elapsing on X. If that isn't a constraint, it's just engineering and biology:

1. If you need to get back to X, without too much time having elapsed on the ship, you just need to go fast. Time dilation keeps the trip short for the travelers.

2. If you are willing to make a one-way trip, you don't need to go fast. There are a couple of approaches:

2a. If you need the original crew to arrive at the destination, then some form of suspended animation (cryogenics?) will do, as well extending your lifespan.

2b. If you don't need the original crew to arrive at the destination, then you can do a generation ship.

If/when we do encounter aliens, I would not at all be surprised if they aren't much more advanced than us. For instance, a civilization that recognized an upcoming planet-wide natural disaster that they could not avert (e.g., big asteroid on collision course), and abandoned their planet on a fleet of generation ships, each ship wandering the galaxy looking for a habitable planet to take over.

That is exactly what I was going to say. Investigating these UFOs would be a waste of money and a waste of time. I remember reading something where people would call to say they have spotted a ufo when they were actually looking at venus (the brightest "star").