Is it possible that it was a targeted snatch, but that Ollie was not the intended target?
My thinking on this possibility: Tony and Mark are not that alike but are both British men of similar height, frame and age; and with a child of same age/size/appearance. Emily was not with Tony at the pool, so he could have been mistaken for a single father. Grey-haired man at pool could have spotted what he thought was the target (ie an English speaking boy with father only) and called in the gang to see if they could get him away. As James was holidaying in the area, they might have been able to make some preparations for what happened after an opportunistic snatch. Major spanner in works as child abduction alert is issued so quickly and makes it plain it is he wrong boy.
The motive would therefore lie in Mark's previous policing, and be somehow connected to a ring with connections across Europe.
And of course whether or not this is the case, Mark might believe it to be so, and part of the weird dynamic between him and Emily links back to him trying to atone for another child being taken in place of his by 'giving' her his child.