The FTSE 100 was down 0.1 percent, with losses led by several stocks trading ex-dividend, while the FTSE 250 added 0.7 percent.
Dublin's main index, considered a measure of Brexit sentiment, jumped 0.6 percent.
The European Union granted Britain a six-month extension to leave the bloc, averting a disorderly exit on Friday and giving Prime Minister Theresa May more time to break the Brexit deadlock at home.
"Rather than worrying about going on holiday over the summer, people can now open their wallets, spend some money, book a holiday and generally start to worry less about a Brexit cliff edge and actually make plans for the next six months," said CMC Markets analyst Michael Hewson.
Blue-chip airline stocks cheered the delay with EasyJet gaining 8.4 percent and IAG adding 5.9 percent on its best day since October 2016.
London-listed shares of German tour operator TUI surged 8.3 percent, with a trader also citing a rating action by Exane.
Domestically focussed financial stocks also outperformed the main index with Lloyds, Barclays and RBS advancing.
Housebuilders, considered more sensitive to Brexit developments, were also buoyed by the extension as well as data showing British house prices improved in March for the first time since July. Persimmon, Taylow Wimpey, Berkeley and Barratt jumped between 2.1-4 percent.
However, losses in heavyweight miners driven by a decline in copper prices stemming from fears of a global economic slowdown and a U.S.-EU trade spat soured the mood on the main bourse.
Precious metals mining company Fresnillo shed 7.2 percent after its quarterly production fell.
Several blue-chip stocks that traded ex-dividend also weighed. ITV, Standard Life Aberdeen, Aviva and Smurfit Kappa fell by between 2.8 and 3.4 percent.
Rating downgrades hurt Marks & Spencer and its online food joint venture partner Ocado. M&S slipped 1.5 percent after a Credit Suisse downgrade and Ocado skidded 2.3 percent after HSBC lowered its rating.
Hewson said the mid-cap index's rally was aided by the fact that the prospect of a cliff edge Brexit or a messy no-deal Brexit had been put off for another six months.
"Investors...can start piling back in to UK stocks, which are I think still fairly cheap," Hewson said.
(Reporting by Yadarisa Shabong and Shashwat Awasthi in Bengaluru; editing by Larry King and Susan Fenton)
By Yadarisa Shabong and Shashwat Awasthi