The FTSE 100 index <.FTSE> closed 0.2 percent up at 7,522.87 points, with Britain's pound on track for its worst week in a year as the Prime Minister hit back at a plot to topple her, saying she would provide "calm leadership" to the country.
"The talk of a leadership plot against the Prime Minister is eroding the pound's value and propping up the FTSE 100," CMC Markets analyst David Madden wrote in a note to clients.
Last month sterling strength helped the FTSE 100 to post a slight decline for September, but the currency has since been losing steam and the index has posted its biggest one-week gain since December last year.
Health stocks, which source a sizeable chunk of their revenue from the United States, were among the biggest gainers. Heavyweights GlaxoSmithKline (>> GlaxoSmithKline) and AstraZeneca (>> AstraZeneca) rose by about 0.3 percent and 1 percent respectively.
British American Tobacco (>> British American Tobacco) was up 1.6 percent and Imperial Brands (>> Imperial Brands) advanced by 0.2 percent.
Shares in budget airline easyJet (>> EasyJet) dropped by 1.6 percent, the biggest FTSE 100 faller, after a price target cut from broker Credit Suisse.
The airline also posted a mixed pre-close update, with analysts pointing to pricing pressures despite easyJet reporting a record summer and saying that it expects to reach the higher end of its profit range.
"Revenue trends are improving, but pricing remains under pressure," Liberum analysts said in a note.
Credit Suisse analysts saw some supportive factors, however.
"With Monarch's failure, Air Berlin's break-up, Alitalia's administration and Ryanair capacity cuts, we expect this confluence of positives must help EZJ (easyJet) pricing," they said in a note.
Shares in building materials company CRH (>> CRH PLC) fell by 1.3 percent after its offer for U.S. Ash Grove Cement Co (>> Ash Grove Cement Co) was surpassed.
(Reporting by Kit Rees and Julien Ponthus; Editing by Keith Weir and David Goodman)
By Kit Rees and Julien Ponthus