No I don't believe you need a dedicated line - the reaction to lactose is an intolerance not an allergy. Lactose intolerance can cause sickness but I have never heard of a fatality associated with lactose ingestion in a lactose intolerant individual.
However if a claim is made re "lactose free" you would need to be pretty confident that your product is just that & consistently too.
To defend against any potential claim there are a number of things that you should do - robust production scheduling & through validated cleaning being two of the controls, as well as end point testing. Relying solely on end point testing is not advisable as, in theory, if your clean is not thorough you could have 'pockets' of contamination which end point testing would not necessarily pick up.
If you can get hold of the M&S 5 point plan to control allergens this is a great resource to walk through the risks & controls associated with an allergen (even if you don't produce for them) and could be used to show that your controls of lactose carryover are thorough.
There is obviously the FDF/BRC guidance notes too - although I don't know how up to date this is - copy attached.
Whilst lactose, being a sugar, not a protein is most definitely not an allergen, if you follow the same controls as you would for an allergen you would have sufficient supporting evidence for a due diligence defence.
BRC-FDF Guidance on Free-From Allergen Claims.pdf 351.17KB
Edited by Lesley.Roberts, 09 January 2019 - 02:11 PM.