In the absence of any further information, it seems likely that these lambs have recently been given some vaccines or other drugs.
As a result, they will not be fit for human consumption until the levels of those drugs have fallen to within acceptable levels.
See the section headed "Withdrawal periods for drugs" in Sheep Medicines (UK source)
Withdrawal periods for drugs (Food producing animals)
The Animals, Meat and Meat Products (Examination for Residues and Maximum Residue Limits) Regulations 1997 control residues of animal medicines in food producing animals. Maximum Residue Limits (MRL) are set which aim to avoid toxicity in man and technical problems for the food producing industries. Under EU legislation the MRL is defined as:
Maximum concentration of residue resulting from administration of an animal medicine which is legally permitted in the Community or recognised as acceptable in or on a food.
Withdrawal periods for meat are listed on the data sheet accompanying the drug and must be strictly observed. Withdrawal periods are defined as:
The time between the last dose given to the animal and the time when the level of residues in the tissues (muscle, liver, kidney, skin/fat) or products (milk, eggs, honey) is lower than or equal to the MRL.
Withdrawal periods are given for time after administration to slaughter (meat production). Where a withdrawal period is not given for a species a minimum of the following "standard" withdrawal periods should be adopted; 28 days for meat. Additionally, some organic food schemes require the doubling or tripling of data sheet and standard withdrawal periods.
The text of the law cited, The Animals and Animal Products (Examination for Residues and Maximum Residue Limits) Regulations 1997, is available online.