The FTSE 100, which had hovered at a near one-month high in the last two sessions, closed with a 0.1% dip. The FTSE 250 <.FTMC> lost 0.2%, led lower by a 9.4% fall in Synthomer after the polymer maker issued a profit warning.
Brexit updates have been the driving force behind a roller-coaster ride for the domestic market over the last few weeks.
While sentiment had improved for battered sectors such as housebuilders <.FTNMX3720> after Prime Minister Boris Johnson sealed a new Brexit deal, caution returned as lawmakers rejected his timeline for the deal's passage.
Parliament forced Johnson to ask for another extension to the departure date, but the European Union has yet to give a go-ahead to the request. Comments from a source close to French President Emmanuel Macron that the delay is not justified at this stage exacerbated worries.
Housebuilders <.FTNMX3720> dipped 0.2% on the sector's fifth straight session in the negative territory.
UK midcaps suffered a bigger hit versus the FTSE 100 on the back of a continued weakness in the pound due to the political tug of war over the course of Brexit.
"Markets may not be fully comfortable with diving head first into riskier assets, until there is greater certainty surrounding the US-China trade conflict and Brexit," FXTM analyst Han Tan said.
Helping limit losses on the main index, Barclays advanced 2.4% after reporting a third-quarter profit that topped analysts' expectations.
WPP, the world's biggest ad firm, jumped 6.1% after it reported a return to quarterly organic sales growth for the first time in over a year, with a new strategy from boss Mark Read helping the company win more work.
"The turnaround at the advertising giant WPP appears to be gaining some traction," AJ Bell investment director Russ Mould said.
Another notable gainer was luxury goods company Burberry, which added 2.1% as results from Gucci parent Kering and Italian jacket maker Moncler allayed some concerns of a major hit to sales from the Hong Kong protests.
National Grid weighed with a 2.4% drop after media outlets reported that New York governor had signalled to cancel the utility's licence if it fails to cooperate with state officials regarding a moratorium on natural gas connections.
(Reporting by Muvija M and Shashwat Awasthi in Bengaluru; Editing by Arun Koyyur and Frances Kerry)
By Muvija M and Shashwat Awasthi