* FTSE 100 up 0.5 pct
* Merlin tops index
* Mediclinic rises after GS ups to "neutral"
* FTSE to flat-line into year-end - Reuters poll (Adds closing prices)
LONDON, Oct 5 (Reuters) - A sharp fall in sterling boosted Britain's main share index on Thursday, with basic resources and oil and gas stocks contributing most to the upswing.
The FTSE 100 closed up 0.5 percent at 7502.79 points after the pound hit a four-week low against the dollar as Theresa May's speech before the Conservative Party thrust her premiership into doubt.
Miners Anglo American, Glencore, Antofagasta and Rio Tinto lifted the index with gains between 1.8 and 3 percent.
Merlin shares posted the best performance of the index with a 3.4 percent rise after a source said the Madame Tussauds owner submitted an offer for parts of Seaworld .
"Historically the group has demonstrated good discipline around M&A," said analysts at Barclays, adding too little was known about any potential transaction to judge it at the moment.
South African private healthcare firm Mediclinic rose 1.6 percent after Goldman Sachs raised it to "neutral", arguing the shares' recent decline took them to a level which better reflected a challenging economic environment in South Africa.
Tesco shares recovered from Wednesday's losses, up 1.4 percent and boosted by UBS raising its price target on the stock.
Britain's biggest energy provider Centrica failed to bounce back from its losses during the previous session when Theresa May announced a cap on energy prices.
Share were down 1 percent but peer SSE took 1 percent back as the sector urges the government to reconsider.
Shares in small-cap DFS fell 4.2 percent after the furniture retailer reported a 13 percent fall in full-year core profits, blaming a "very challenging" UK market.
"While market conditions are currently challenging, we believe DFS is in a strong position to gain market share against a tough backdrop," said Berenberg consumer discretionaries analyst Victoria Maigrot.
A Reuters poll published on Thursday found investors see the FTSE hardly budging in the last quarter of the year as concerns over Brexit negotiations grow and earnings expectations soften.
"The economic backdrop for the UK continues to look concerning, with negative real wage growth, low household savings, high consumer debt and falling consumer confidence," said Edward Park, investment director at Brooks Macdonald.
However, he added: "The fact the market is positioned for a tough domestic backdrop stops us from becoming outright bearish as a lot of the potential risk is priced in."
(Reporting by Helen Reid and Julien Ponthus; Editing by Robin Pomeroy)