The sources said the deal is expected to include an increase in the percentage of the UK's state-run National Health Service (NHS) budget that is spent on medicines.
It will also involve a major change in the value appraisal framework at NICE, a UK government body that determines whether new drugs are cost-effective for the NHS, the sources said.
NICE's "quality-adjusted life year" measures the cost of a treatment for each healthy year it enables for a patient, with the upper threshold being 30,000 pounds ($A60,600) per year.
That threshold is set to be increased by 25 per cent, the sources said, speaking on condition of anonymity to discuss confidential talks between the governments.
The White House and the UK government did not immediately respond to requests for comment.
UK Science Minister Patrick Vallance had earlier flagged that trade talks with the United States would likely lead to higher government spending on some medicines.
He told MPs in October that "some degree of price increase is inevitable" for new, innovative medicines under the talks although existing drugs and generics may not have the same rises.
US President Donald Trump has pressed the UK and the rest of Europe to pay more for US medicines, part of his push for US medicine costs to be brought more in line with those paid in other wealthy countries.
Major pharmaceutical firms have meanwhile delayed investment in the UK including AstraZeneca, the largest firm on the London Stock Exchange by market value, citing a tough operating environment.
The two countries agreed in May to seek "significantly preferential treatment outcomes on pharmaceuticals," with a commitment that the UK tries to improve the environment for pharma firms operating in the country.
