On the Thomsonfly website you will see that there are telephone numbers given to call from various countries. For instance they give the number to call from Holland as 0207121384.  You can call Dutch landlines from UK landlines starting from 0.5p per minute (minimum charge 5p).  Refer to
 
http://www.moneysavingexpert.com/intcallchecker/  

On this website you will find various BT access numbers for Dutch landlines with the per minute price.

You will have to prefix the Dutch number given by Thomson with 0031, i.e. the actual number to dial (after the access number) will be 0031207121384