Today a customer wanted to buy four packets of painkillers, two packs of ibuprofen and two packs of paracetamol. I couldn't allow more than 2 packs per transaction so I sold her two of them on one transaction, she paid for it and then I split the transaction so she could pay for the other two on a separate one. Is this against the company policy and will I be fired if I get caught? The woman looked mature enough and looked unlikely to be suspected of taking an overdose later on.

This is a world site. I for one have no idea who B&M is. Ask the manager for a copy of policy.

In the US, I know of no limits on the purchase of OTC ibuprofin and acetaminophen in any given drug stores.

As this is a UK issue I can answer.
Rather than simply being store policy, this is a national law concerning the volumes of painkillers that can be sold in one VISIT.
You should not have split the transaction as you did as they are both within the same visit.
What will happen is up to your line manager.