The Ftse 100 was 26.2 points higher at 6695.4 after the value of the microchip designer jumped more than 40%, or 486p, to 1675p, after the Japanese company tabled a £24bn deal.
SoftBank pledged to double the tech giant’s 3,000-strong UK workforce over the next five years, while holding on to its Cambridge headquarters and its existing management team.
The deal is the biggest ever Asian investment in the UK and values the technology firm at 1700p per share, a 43% premium on Friday’s closing share price of 1189p.
Sterling was up 0.7% against the dollar at $1.329, while the pound also rose 0.3% against the euro at €1.199.
Across Europe, Germany’s Dax was slightly down, while the CAC 40 in France edged 0.3% lower.
The price of oil took a tumble after an economic report by market intelligence firm Genscape pointed to rising US stocks of crude.
Brent crude was down 1.8% or 86 cents to 46.75 US dollars a barrel.
In stocks, travel firms were under pressure amid fears that the failed coup in Turkey would have a damaging impact on the tourism industry. TUI group was among the biggest fallers, down 19p to 942.5p, while easyJet edged down 2p to 1138p.
However, housebuilders were enjoying a lift; Taylor Wimpey was up 3.7p to 147.7p, Charles Church-builder Persimmon lifted 34p to 1605p, and Berkeley Group rose 56p to 2680p.
The biggest risers on the Ftse were ARM Holdings, up 486p to 1675p; Royal Bank of Scotland, up 5.8p to 189.8p; and Marks and Spencer, up 9.4p to 334.7p.
The biggest fallers were BHP Billiton, down 19.7p to 976.8p; BP, down 7.9p to 449.3p; and BT Group, down 5p to 395p.