* FTSE 100 up 4.5 pct in May, best month of the year
* Slips on the day as sterling swings keep investors away
* Diageo, BATS strength not enough to offset weak miners
* Glencore, RIO, BHP slump more than 2% as iron ore slumps (Updates to close)
LONDON, May 31 (Reuters) - Britain's FTSE 100 ended its strongest month of the year on a quiet note on Wednesday as a choppy day for sterling, thanks to an increasingly cloudy picture painted by polls about next week's general election, gave investors little reason to chase the rally.
The UK bluechip index, dominated by dividend-paying exporters whose profits benefit from a weak local currency, hit a record high earlier in the session but lost those gains to finish the day slightly lower.
Those moves closely tracked sterling which was knocked around by various polls that pointed to a range of possible outcomes -- from a hung parliament to a solid Theresa May-led Conservative win.
"You really cant run a portfolio around poll results," said Eric Moore, a portfolio manager at Miton Group, who added he had not changed his portfolio much and remained wary of UK consumer stocks.
"If there is a hung parliament youve got to be quite worried for the pound and the UK 10-year yield on gilts," Moore warned.
A poll from YouGov published overnight on the distribution of seats after the June 8 vote pointed to a loss of 20 seats for Prime Minister Theresa May's Conservative party that would leave her short of an overall majority in a parliament where she lacks potential coalition partners.
Other projections still show May would win soundly though the YouGov poll was the latest hit to markets which just two weeks ago were pricing in a Conservative landslide.
The sharpest swings, as has been the case since last year's Brexit referendum, were on sterling which hit an almost six-week low in morning trade only to bounce back later in the session.
Political uncertainty has had less of a direct impact on the FTSE 100 which posted a 4.5 percent gain in May, its best monthly showing this year.
Those gains were driven largely by strength in shares of big multinational companies such as British American Tobacco , Diageo and HSBC, which generate the majority of their revenue outside the UK.
They rose again on Wednesday though their gains were not enough to offset weakness in mining companies that suffered from a slump in the price of iron ore.
Commodity-related stocks, usually also beneficiaries from a weak pound, suffered as a supply glut and selling by Chinese speculators spurred the sharpest rout in iron prices this year.
Bellwethers Glencore, Rio Tinto and BHP Billiton fell more than 2 percent.
Elsewhere, shares of payment processor Worldpay fell 1.5 percent after Barclays downgraded the stock to "neutral," adding it expected the company was likely to miss certain growth targets.
Among mid-caps, online food delivery service Just Eat rose 3 percent to a record high as local broker Peel Hunt started coverage on the stock with a "buy" rating. Just Eat shares are now up more than 150 percent since their listing. (Reporting by Vikram Subhedar and Tricia Wright; Editing by Keith Weir)