A rally in pharmaceutical stocks led by industry giants GlaxoSmithKline and AstraZeneca helped the FTSE 100 outshine most global peers on Wednesday while investors waited for the outcome of the US Federal Reserve's policy meeting.

The FTSE 100 reversed early losses to close up 0.3%, with the pharma sub-index scaling an all-time high, up 2.5% after GSK again upgraded its 2019 targets.

The midcap index ended 0.3% lower as more domestically-focused companies braced for a wide array of Brexit possibilities after parliament approved a December election, leaving the next steps on Britain's departure from the European Union unclear.

Moves on both indexes were however subdued with investors waiting for the Fed to announce its policy decision.

The world's largest central bank is expected to lower interest rates for the third time this year as it looks to soothe an economy bruised by the protracted trade war with China.

UK Prime Minister Boris Johnson has won parliamentary support for an election on Dec. 12 as he seeks to break political deadlock after lawmakers forced his hand to delay to Brexit.

The FTSE 100 has gained just 15% since the UK voted to leave the EU in mid-2016, a far cry from the 50% rise for its Wall Street counterpart, the Dow, and reflecting the cautious view on UK equities while political uncertainty drags on.

The FTSE recorded a third month of losses for 2019 in October, with exporter stocks knocked by a firmer pound as no-deal Brexit fears abated.

GSK shares rose 2.5% to their highest in more than six years after the company hiked its annual profit forecast with sales of its shingles vaccine topping expectations.

Rival AstraZeneca was the biggest support to the main bourse with a near 3% gain after agreeing to sell the European rights for its schizophrenia drug to Cheplapharm.

Standard Chartered added 2.6% after a forecast-beating rise in quarterly profit, even though it flagged headwinds from a likely drop in global growth and lower interest rates.

Next dropped 3% after the retailer said sales in September were hit by unusually warm weather and small-cap passport maker De La Rue plunged 20% to its lowest in two decades after issuing another profit warning.

Among other midcaps, medical products maker ConvaTec

jumped 11% after quarterly revenue came in above market expectations and Computacenter advanced 9% following upbeat results for the last three months.