Why can't some airports handle heavy aircraft while others do it easily (same runway length)?
There are a few possible reasons Bournemouth (EGHH) can support a Boeing 787 but Kherson (UKOH) can't. Runway length is definitely not the issue. The runway at EGHH is 2271 metres long, the one at UKOH is 2500 metres long.

The runway at UKOH is slightly narrower, though: 42 metres compared to 46 metres at EGHH. Whether this has operational significance, I do not know.

Another thing to compare is the strength of the pavement, which is expressed as a Pavement Classification Number, or PCN.

The PCN of the runway at EGHH is 46/F/B/X/U, at UKOH it is 28/R/C/X/T. Those values can be decoded like this:


EGHH: Load carrying capacity: 46, flexible pavement (typically asphalt), high strength, high tire pressure supported.
UKOH: Load carrying capacity 28, rigid pavement (typically concrete), low strength, high tire pressure supported.


This is a bit technical, but what it means, in essence, is that the pavement at UKOH is not nearly as strong as that at EGHH. The 787 has a max takeoff mass of around 230 tons, so it needs some pretty strong pavement to support it.

Finally, the rescue and fire fighting services provided at the two airports is different. EGHH provides up to CAT 9 (capable of handling aircraft up to 76 metres long and 7 metres wide (fuselage width)). UKOH provides only CAT 7 (capable of handling aircraft up to 49 metres long and 5 metres wide.

The 787-8 (the smallest of the 787 variants) is 56,7 metres long and 5,77 metres wide. So most likely, what's keeping Kherson Airport from handling 787's is a big investment in new pavement, and maybe the need to buy a few extra fire trucks.

Then of course, there's also the whole question about capacity of the terminal building, size of gates and various equipment (push back trucks, stairs, jet bridges etc.). Those things are a bit harder to compare directly, though.