* FTSE 100, FTSE 250 down 0.2%
* China's outlook weighs on oil and gas firms
* Trade tensions, Brexit uncertainty curb risk sentiment
* GSK rises after ovarian cancer study results (Adds news items, updates to closing prices)
Sept 30 (Reuters) - Britain's FTSE 100 index closed lower on Monday, weighed by oil majors as concerns over global growth resurfaced due to a weak economic outlook for the world's largest crude importer, China, amid simmering trade tensions with the United States.
The exporter-heavy FTSE 100 index fell 0.2%, slightly off its near two-month high, and the mid-cap FTSE 250 index dropped by the same margin.
Shell and BP were the biggest drags on the main bourse, tracking a fall in crude oil prices.
Still, the FTSE 100 index had its best month since June, bouncing back from losses in August that were triggered by escalating trade jitters and accompanying fears of a global recession, as well as sustained Brexit anxiety. The domestically-focussed FTSE 250 registered its biggest monthly rise since April.
On a quarterly basis, the main bourse was marginally down, while the mid-cap index enjoyed its third consecutive quarter in positive territory.
GlaxoSmithKline and AstraZeneca both reported trial results on Saturday that will likely make their competing drugs available to a wider group of ovarian cancer patients, possibly helping GSK catch its rival in a highly contested drug class.
Shares of GlaxoSmithKline rose 1%, while and AstraZeneca dipped 1.7% after nine consecutive sessions in the black.
Global miners also contributed to the session's losses with a 1% fall as sentiment around U.S.-China trade remained uneasy.
White House trade adviser Peter Navarro on Monday dismissed reports that Washington was considering de-listing Chinese companies from U.S. stock exchanges.
With just a month left until Britain is scheduled to leave the European Union, there was very little action on the Brexit front and market participants were left waiting for what may happen next in Britain's already chaotic Brexit process.
"Investors have two of the horsemen front and centre this week - it's a case of trade tensions ad nauseum, Brexit ad infinitum," Markets.com analyst Neil Wilson said.
Among individual stocks, Premier Inn owner Whitbread dropped 3.9% after Barclays downgraded it to 'equal weight', saying it sees more downside than upside risks.
Small-cap Metro Bank added 3.8% after a report that activists led by Elliott Advisors were mulling taking a stake in the lender.
Mid-cap HomeServe rose up 1.8% after RBC raised its rating and nearly doubled its price target on the home repairs provider, while Sirius Minerals more than recouped losses seen last week with a 37% jump. (Reporting by Yadarisa Shabong and Shashwat Awasthi in Bengaluru; additional reporting by Indranil Sarkar; Editing by Bernard Orr and Alex Richardson)