The law does not specifically allow you to delay your return to work from maternity leave or to take special leave once back at work in order to continue breastfeeding, but the law can still help.
You should ensure that your employer has written notification that you are breastfeeding so that in the health and safety 'risk assessment' they can take account of the particular risks to you and your baby while you are breastfeeding. If your working conditions prevent you from continuing to breastfeed, your baby's health will be put at risk, so your employer should allow you to make reasonable adjustments to your job such as breaks to breastfeed or express milk, or a shorter working day. Your employer also has a legal duty to provide 'suitable facilities' for a breastfeeding mother to rest.
If your job is so stressful or tiring that even with adjustments you would be unable to continue breastfeeding then you have the right to be transferred to a different job that was compatible with breastfeeding, or (in theory at least) to be suspended on full pay.
If your employer is not complying with these legal provisions, is treating you badly, or is threatening dismissal, take advice. In many cases, sex discrimination, unfair dismissal or protection from detriment rights will apply as well.