UPDATE 2-UK shares ride Sino-U.S. trade high; election result watched

* FTSE 100 up 1%, FTSE 250 up 0.7%

* First exit poll for election result at 2200 GMT

* Trump says Washington close to "big deal" with Beijing

* WPP rises on share buyback agreement

* John Laing lags mid-caps (Adds news items, analyst comment, graphic, updates to closing prices)

By Muvija M and Shashwat Awasthi

Dec 12 (Reuters) - UK shares leapt on Thursday as U.S. President Donald Trump said the United States and China were very close to a trade deal, providing a shot in the arm to what had been a wait-and-watch session for markets with Britons voting in an election.

The FTSE 100 rose 1%, driven by a 3% jump in HSBC and gains in miners and oil stocks on the back of Trump's comment, which came days before tit-for-tat tariffs are set to take effect.

The FTSE 250 broke a three-day losing streak as it added 0.7%. Though initially tentative, gains were cemented after Trump's tweet on trade and a report that the U.S. had offered to cancel the imminent tariffs on Chinese goods.

The more domestically-inclined index lagged the blue-chip bourse, however, as traders exercised caution with voting under way in the general election which could decide the fate of Brexit.

Intra-day trading volume on the FTSE 100 was at its lowest since May 1 and the second-lowest so far this year.

"The moves are relatively modest. At this stage, people are waiting for the exit polls and the election results in the due course... We are seeing a FTSE 100 out-performance but in the wider scheme of things, it is modest," Raymond James analyst Chris Bailey said.

Polls in the run-up to the election had indicated that Boris Johnson's Conservative Party would win a majority, but recent estimates have shown that its lead over the opposition Labour Party narrow, stoking worries of a hung parliament.

Despite earlier holding firm, sterling slipped as traders booked in profits, helping prop up exporter stocks, while utilities like Severn Trent, which run the risk of re-nationalisation under a Labour government, dipped.

News-driven gainers saw ad firm WPP rise 3.2% after a share repurchase agreement.

Earlier in the session, sentiment was supported by the U.S. Federal Reserve signalling that interest rates would likely remain accommodative.

BlackRock's Chief Investment Officer of Global Fixed Income, Rick Rieder, called it "a very good policy prescription".

John Laing fell 10% among midcaps on its worst day ever, after the infrastructure firm said it expects annual net asset value to miss market estimates.

Balfour Beatty rose 4.3% after forecasting annual profit ahead of its estimates and electricals retailer Dixons Carphone jumped 7% as it maintained its full-year outlook.

(Reporting by Muvija M and Shashwat Awasthi in Bengaluru; Editing by Arun Koyyur and Angus MacSwan)