The FTSE 100 rose 0.7pc, scaling its highest level since Aug. 29, boosted by shares of companies that book a major chunk of their revenue overseas. The FTSE 250 added 0.6pc.
The pound struggled for traction after Bank of England Governor Mark Carney said on Tuesday that the global trade war and a 'no-deal' Brexit may warrant a near-term policy response, which markets interpreted as a dovish signal.
The currency's woes were compounded after Britain's composite purchasing managers' index last month slipped into contraction territory for the first time since mid-2016, highlighting the mounting toll Brexit uncertainties have taken on the economy.
Healthcare firms, consumer companies and utilities - a mix of stocks that benefit from a weaker pound and are considered "defensive" - were among the biggest boosts to the main index.
"There is certainly general interest in defensive names but I think this is a global theme," Raymond James analyst Chris Bailey said, while Cityindex analyst Ken Odeluga noted the defensive rotation had "more to do with the nuance of sentiment on the day".
Appetite for policy easing was stoked last month when investors bet the US Federal Reserve would cut interest rates in response to slowing global economic growth, which helped the FTSE 100 enjoy its best month since January.
AIM-listed Telford Homes climbed 13pc to 356 pence, on their best day in almost 10 years, after US real estate firm CBRE said it would buy the British residential property developer for 350 pence a share in cash.
Shares of Flutter Entertainment surged 11.5pc to their best day in more than a year. Though speculation about a possible buyout persisted, the company said it was unaware of a reason for the jump in its share price.
Supporting the mid-cap index was cyber security firm Sophos , which advanced 8.2pc, while peer Avast rose 2.5pc, after sources said chipmaker Broadcom was in advanced talks to buy Symantec.
Drinks makers fizzed amid hopes of a favourable review into the so-called "sin taxes" on products high in salt, fat and sugar to be promised by PM frontrunner Boris Johnson.