The blue-chip FTSE 100 index ended 0.1% lower at 10,363.93 points, while the midcap FTSE 250 climbed 0.3%.
AstraZeneca shed 3.1% after a U.S. Food and Drug Administration advisory panel voted against recommending an experimental breast cancer treatment the drugmaker has said is central to its long-term growth ambitions.
NatWest reported a 12% rise in first-quarter profit, but its shares fell 3.4% after the lender's non-interest income was 7% below analysts' forecasts.
Heavyweight energy stocks Shell and BP were among the biggest drags on the blue-chip index, tracking weakness in crude oil prices.
Tehran has submitted its latest proposal for negotiations with the United States, but it is unclear whether it can break a deadlock in efforts to end the Iran war.
The gainers included Pearson, which rose 3.1% after the education company said demand for its virtual learning products drove a 4% rise in first-quarter underlying group sales.
Consumer staples giant Unilever jumped 2.6% to hit a one-month high, while engineering company Rolls-Royce added 1.5%, extending gains a day after it reiterated its profit outlook.
Spirits maker Diageo ended 0.7% higher. It had risen by as much as 2.8% after U.S. President Donald Trump said he was removing tariffs on UK-made whisky.
The FTSE 100 clocked marginal weekly declines, taking its consecutive weekly losses to a third after mixed corporate earnings this week hit sentiment.
Trading volumes on London exchanges were well below their 20-day moving average, as most markets across Europe were closed on Friday and Britain has a bank holiday on Monday.
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