The main bourse climbed 0.67 percent by 0945 GMT and the FTSE 250 was up 0.2 percent.
GlaxoSmithKline surged 7.1 percent to take the top spot on the FTSE 100 after saying it would combine its consumer health business with Pfizer's in a joint venture with sales of 9.8 billion pounds ($12.41 billion).
"We view today’s move as further evidence of GSK’s renewed focus on Pharmaceuticals R&D and believe separation of the Group provides an opportunity for further value delivery from the individual entities," said Shore Capital analysts.
Healthcare stocks were the biggest boost to the main index.
In the red was postal services group Royal Mail, down 2.4 percent after weak results from FedEx in the U.S. dented sentiment across the post and logistics sector.
After poor results from ASOS on Monday sent the stock down 38 percent and sent shivers across the retail sector, FedEx provided the latest window into the effects of weaker consumer sentiment impacting not only retailers but also the companies charged with delivering purchases.
Deutsche Post shares also fell 4.9 percent as FedEx blamed a weaker European economy for its profit warning.
Royal Mail's slide comes just before the century-old UK firm loses its FTSE 100 status later this week.
On the Brexit front, the government said it would implement plans for a no-deal scenario in full, even as Prime Minister Theresa May is set to bring her divorce deal back to the parliament in mid-January.
With the March 29 exit date fast approaching, there was still little clarity on how things would pan out with factions pressing for different options for future ties, leaving without a deal or remaining in the EU.
Investors concentrated instead on the Fed decision.
"Today there's a bit of relief from Brexit although we've got the strong mood music coming out of Downing Street that everyone's got to batten down and prepare for no-deal," said Jim Wood-Smith, head of research at Hawksmoor Investment Management.
"The markets know that and they are much more focused on the likely outcome from the Fed this evening."
Among smaller stocks dealmaking was also a driver, with airline Flybe Group jumping 11 percent after Virgin Atlantic said talks about a potential takeover offer were ongoing.
Flybe has roughly doubled in market value since Sky News broke the news on the takeover talks in late November - but the struggling airline has still sunk 43 percent this year as profit warnings took their toll on the share price.
Shares in online gambling company 888 Holdings rose 6.5 percent, the best mid-cap performers after it said positive trends in its UK revenue continued over the second half of the year.
Small-cap Gulf Marine Services plunged 69 percent after the support vessels maker said it expects to be in breach of some banking covenants at the end of this year.
AIM-listed YU Group, a gas and electricity supplier, lost nearly a third of its value and hit an all-time low after it said the UK's financial watchdog was planning to investigate some of the company's announcements from this year.
($1 = 0.7896 pounds)
(Reporting by Muvija M and Shashwat Awasthi in Bengaluru, Editing by Helen Reid)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)