You should get permission from the Enviroment agency's of the source country and the UK, in the UK you need to contact DEFRA. You also need to contact customs before bringing ants back.
Many people send species thru the post, which is a grey area, however in Europe only certain species (that are considered invasive and/or destructive to wild life) are restricted, its always best to contact DEFRA first
The law in the UK regarding imported insects is that as long as you dont release ANY of them into the wild, even ones from as close as france, then its legal to import them.
However you must be very careful to help protect our ecosystem, so use good packaging always