* FTSE 100 up 0.6 pct with pro-unionists leading in Scotland
* Also boosting equities: U.S. Fed renews zero rate pledge
* TUI Travel gets lift from Morgan Stanley upgrade
* Monitise tumbles as Visa explores options for stake
By Tricia Wright
LONDON, Sept 18 (Reuters) - Britain's top shares rose on Thursday as investors started to price in a vote by Scotland to remain in the United Kingdom, with further support coming from expectations that interest rates will stay low in the United States for an extended period.
As voting on Scottish independence began at 0600 GMT on Thursday, various surveys showed support for union at 52 percent and for independence at 48 percent. Another showed unionist support at 51 percent and separatist at 49 percent. Yet another put the gap at 53 percent and 47 percent.
"The polling stations in Scotland may be open for a good few hours yet, but the City appears to have already concluded that this is a win for the 'No' campaign," said Tony Cross, a market analyst at Trustnet Direct. "I think a 'No' will send stocks much higher tomorrow."
A Thomson Reuters basket of 12 stocks listed on Britain's FTSE 350 index, headquartered in Scotland, has slowly risen over the last two weeks. (link.reuters.com/wej72w)
The stocks have recovered from lows at the start of the month, after plunging when a poll showed a surprising lead for the pro-independence side.
Cavendish Asset Management fund manager Paul Mumford said he had used the earlier pullback on Scottish-exposed stocks to buy those shares cheap.
"I personally think that there will probably be a 'No' vote. If there's a 'No' vote, it removes the uncertainty hanging over those stocks," said Mumford.
Mumford added to holdings in Scotland-headquartered distribution group John Menzies and North Sea oil-focused groups Hurricane Energy and Ithaca Energy . Although not included on the FTSE 350 index, they are part of the broader London stock market.
Equities across Europe temporarily trimmed gains when the European Central Bank handed out a below-forecast 82.6 billion euros ($106.38 billion) in its first offering of four-year loans to banks.
But traders said this was offset by expectations that a small allocation might prompt the ECB to take further stimulus measures, such as buying assets to print money - a tool known as quantitative easing (QE).
The FTSE 100 closed up 38.39 points, or 0.6 percent, at 6,819.29 points. It was also propped up by the U.S. Federal Reserve's reiteration on Wednesday of its pledge to keep ultra-low interest rates for a "considerable time".
TUI Travel jumped 4.7 percent to the top of the blue-chip leader board after Morgan Stanley upgraded its rating on the stock to "overweight" from "equal-weight" following the tour operator's recent merger agreement with majority owner TUI AG.
Volume in TUI Travel stood at more than three times its 90-day daily average volume, against that for the FTSE 100 at 108 percent.
Among other notable movers, small-cap Monitise tumbled almost 35 percent after Visa Inc said it was exploring options for its 5.5 percent stake in the British mobile banking technology company. Its trading volume was eight times its 90-day daily average. (1 US dollar = 0.7765 euro) (Additional reporting by Sudip Kar-Gupta)