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* Shell leads oil & gas stocks lower

* Countryside hit by weak earnings outlook

* 888 Holdings surges on reduced William Hill deal value

* FTSE 100 down 0.5%, FTSE 250 off 0.3%

April 7 (Reuters) - Britain's FTSE 100 slid on Thursday, weighed down by oil major Shell after it flagged a bigger writedown following its decision to exit Russia, while a jump in shares of gambling firm 888 Holdings helped limit some losses on the midcap index.

The blue-chip index closed 0.5% lower, with Shell falling 2.1% as it raised its Russia writedown to as much as $5 billion, more than previously disclosed.

Broadly, economy-linked sectors such as oil and gas, miners, banks and insurers fell on concerns over slower economic growth following hawkish signals from the U.S. Federal Reserve and amid the war in Ukraine that has elevated inflation.

"The Fed minutes confirmed the feeling in markets, which was that 50 bp rate hikes were on the table and that quantitative tightening will begin in May even though the market believes economic growth will be impacted," said Edward Park, chief investment officer at Brooks Macdonald Asset Management.

A Bank of England survey showed that British companies' expectations for output price inflation in the coming 12 months had hit a new high of 5.9% last month, while 48.5% of companies thought the Russia-Ukraine conflict would lower sales.

The domestically focused FTSE 250 midcap index slipped 0.3%, dragged down by Countryside Partnerships. Shares of the housebuilder tumbled 14.6% after it warned of lower annual profit following a business review of its operations.

Overall, the sectoral index fell 3.5% after mortgage lender Halifax said British house prices grew strongly in March but the cost-of-living crisis clouded the outlook.

"We've got a fairly conservative view towards the housing sector. If we do see a turndown in consumer confidence, that will impact housing and if actually consumer demand remains quite robust and allows the BoE to carry on raising rates, that's also going to be negative for house prices," said Park.

888 Holdings surged 16.8% after the gambling firm and U.S.-based Caesars Entertainment cut the deal value of William Hill's non-U.S. assets, allowing a smaller equity raise to fund the deal.

Peer Entain dropped 4.1% as its online sales fell for a second straight quarter. (Reporting by Sruthi Shankar and Devik Jain in Bengaluru; editing by Subhranshu Sahu and Bernadette Baum)