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The FTSE 100 was up 0.4% to 6,570.94 by midday, with gains slightly undercut by sterling strength as Brexit negotiations continued past Sunday's apparent deadline.
Other European markets were even more strongly in the black with Germany's DAX index up 1.3%.
Pharmaceutical giant AstraZeneca dropped 6.6% to £76.20 on confirming it had agreed to shell out $39 billion to acquire Boston-based Alexion Pharmaceuticals, bolstering its presence in immunology.
Separately, AstraZeneca also scored regulatory approvals in Europe for respiratory disease and breast cancer treatments.
Video games developer Codemasters soared 20.4% to 643p after it accepted a £945 million takeover offer from Electronic Arts that had trumped a rival bid from Take-Two Interactive.
EA offered 604p per Codemasters share, well above the implied 528p value of a cash and shares offer lobbed last month by Take-Two.
Consumer goods giant Unilever fell 0.6% to £43.94 on announcing that it would put its climate action plan, which includes achieving zero net emissions from its operations by 2030, to a shareholder vote.
Unilever said the vote, although only non-binding and advisory in nature, would be the first time a major global company had voluntarily committed to put its climate plans before a shareholder vote.
Piping system manufacturer Polypipe jumped 12.7% to 515p, having upgraded its annual profit guidance, citing a continued recovery in demand.
Polypipe's underlying operating profit for the year to 31 December was now expected at around £40 million, compared to the current consensus range of £35 million-£37 million.
Real estate portal OnTheMarket rallied 6.1% to 130p as it, too, upgraded its annual guidance.
OnTheMarket's revenue for the year through January was now expected to be no less than £22.5 million, with adjusted operating profit no less than £1.5 million.
Recruitment firm SThree firmed 3.1% to 302p, even as its annual fee revenue dropped 8% after a positive performance in the US was more than offset by weakness in Europe and Asia.
On the brighter side, SThree said its performance had improved in the fourth quarter versus the third. Future dividend payments were 'under active review', it added.