Where is the flight going to get to Khao Lak? And ... no Khao Lak is not an island, but is located on the mainland.
However, the nearest airport is on an island, the International Airport Phuket (HKT). The island of Phuket is connected to the mainland by the Sarasin Bridge in the far north.
The airport is 80 km south of Khao Lak, an hour's drive away.
Since there are few direct flights from Europe, you are most likely to fly
via Bangkok or one of the Arab or Asian cities.
Very few flights, e.g. with Singapore Airlines or Air Asia, arrive in Krabi,
Approx. 170 km southeast of Khao Lak, 2 1/2 hours by taxi.
The Ranong or Surat Thani airports are only a temporary solution, approx. 180 km north or east (a good 2 1/2 hours by car through the mountains).
If the luggage is not checked through to Phuket, you should plan at least one hour more buffer at the usual transit time of 1.5 hours.
Please remember to drink a lot on the plane. Doctors recommend 0.25 liters of liquid per flight hour, which is almost 3 liters from Europe to Bangkok !!! However, alcohol should be avoided because alcohol has a much stronger effect in the compressed air of the aircraft cabin than on the ground.
Older people in particular can swell their legs when flying, which is a risk of thrombosis. You can prevent this with compression stockings. Friends of ours have had very good experiences with "Travel socks". Even after 20 hours from Phuket to Germany, there was no swelling of the legs.