The main index, whose companies get more than two-thirds of their profit from abroad, was up 0.3% at 0816 GMT, while the more domestically-focussed FTSE 250 was up 0.1%.
The indexes outshone European and Asian counterparts, where confidence was hit by media reports the United States is considering Huawei-like sanctions on Chinese video surveillance firm Hikvision.
A slide in sterling led internationally-exposed companies British American Tobacco, Unilever and Diageo to be among the biggest risers on the FTSE-100.
In contrast, blue-chip housebuilders including Persimmon and Barratt lost more than 2% after gaining in the previous session when May unveiled her new Brexit deal.
M&S slid 5% to a more than four-month low after it priced a rights issue at a big discount to Tuesday’s close. It also reported a third straight decline in annual profit, emphasising the pain of its latest turnaround plan.
SSE slipped 2% after reporting a slump in annual earnings and warning of an uncertain outlook due to the opposition Labour party’s plans to renationalise energy networks.
Strong earnings reports lifted the mid-cap index, despite the pound’s weakness.
Financial trading platform IG Group surged 9.1% after it unveiled a plan to drive growth even as it forecast a drop in full-year net trading revenue and operating profit.
That helped rivals Plus500 and CMC Markets to gains of 4.4% and 2.1% respectively.
Royal Mail, the former postal monopoly, gained 7.2% – its biggest one-day rise in 5-1/2 years – as investors focussed on the company’s new five-year turnaround drive instead of a dividend cut.
Pets at Home jumped 10% after reporting better-than-expected revenue and forecasting higher earnings for 2020.
Weighing on the index was engineering services group Babcock, which tumbled 9.1% after saying it expected revenue and underlying operating profit to fall in 2019/2020.
(For graphic on online trading platforms lag the wider index due to regulatory woes, click tmsnrt.rs/2WlhmfX)
Reporting by Muvija M and Shashwat Awasthi in Bengaluru; Editing by Andrew Heavens and Mark Potter