By Carla Mozee, MarketWatch

U.K. stocks gained Tuesday, with RSA Insurance Group Inc. rallying on word of possible takeover bid, but Royal Mail PLC shares fell following a regulator's rebuke over practices at the postal-delivery company.

The FTSE 100 rose 0.4% to 6,529.53, on track for its first advance in six sessions.

RSA (RSA.LN) surged 11% after Zurich Insurance Group AG (ZURN.VX) said it's "evaluating a possible offer" (http://www.marketwatch.com/story/zurich-confirms-its-considering-an-offer-for-rsa-2015-07-28) for the British firm. The Swiss insurer said it can't, however, make assurances that an offer will be presented.

Meanwhile, Hikma Pharmaceuticals PLC (HIK.LN) climbed 5.9% after the company said it's buying Roxane Laboratories Inc (http://www.marketwatch.com/story/hikma-to-buy-roxane-labs-for-up-to-265-billion-2015-07-28). and Boehringer Ingelheim Roxane Inc. for up to $2.65 billion in cash and shares. The deal is expected to push Hikma's position in the U.S. generic drugs market.

In other deal news, GKN PLC (GKN.LN) shares popped up 5.4% after the engineering company said it will buy Fokker Technologies Group BV (http://www.marketwatch.com/story/gkn-to-buy-fokker-as-it-bets-more-on-aerospace-2015-07-28) for an enterprise value of 706 million euros ($779.98 million) from private equity owner Arle Capital. "Aerospace has been our top priority for acquisitions," said GKN Chief Executive Nigel Stein in a statement.

But at the bottom of the FTSE 100 was Royal Mail (RMG.LN). Shares fell 2.5% after regulator the U.K. Office of Communications, or Ofcom, (http://www.marketwatch.com/story/royal-mail-breached-competition-law-says-ofcom-2015-07-28) said the company's changes to wholesale prices for bulk mail-delivery services, or access services, breached competition law by discriminating against rival operators.

BP PLC (BP.LN) shares, meanwhile, turned higher by 0.8%. The oil major swung to a second-quarter loss (http://www.marketwatch.com/story/bp-swings-to-quarterly-loss-on-lower-oil-prices-2015-07-28), hurt by lower oil prices and a multibillion-dollar charge relating to the recent deal to settle claims in the U.S. over the 2010 Deepwater Horizon disaster.

Investors will be watching later for the release of the first reading on U.K. gross domestic product in the second quarter. Ahead of the report, the pound was buying $1.5542 versus $1.5559 late Monday in New York.

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