A fall in the value of sterling following disappointing UK services data and oil major BP's earnings beat helped the FTSE 100 rally 1.4% to 7,135 at around midday.
The UK services sector could be on the brink of contraction after Markit's Purchasing Managers' Index fell from 51.2 in December to 50.1 in January. A reading of over 50 implies growth while one under 50 suggests contraction.
Among the blue-chip risers were Royal Dutch Shell, up 1.7% at £24.73, and miners Antofagasta and Glencore.
Brent crude oil was stable at $62.54 per barrel.
MID AND LARGE CAP RISERS AND FALLERS
BP rallied 5.4% after the energy giant more than doubled its annual profit amid rising oil and gas output boosted by its purchase of US shale assets from BHP.
Online grocery retailer Ocado pared earlier falls to advance 0.9% as it posted a £44.9m annual loss amid continued investment in its technology platform. Sales rose 12%.
Indivior tumbled 11.2% on news that it failed to appeal against a decision that would allow rival Dr Reddy's Laboratories to sell a generic version of its opioid addiction treatment.
Fellow pharmaceutical company AstraZeneca was having a better morning, rising 1.2% as its treatment for lower respiratory tract infection received priority status from European regulators.
Irish distribution specialist DCC revealed operating profit in the 31 December was 'significantly ahead' of last year, helping the stock gain 1.1% to £64.70.
Industrial and residential property investor St. Modwen Properties gained 0.6% to 406p after posting a 4.3% rise in its net asset value per share in 2018 and boosting its dividend.
SMALL CAP RISERS AND FALLERS
Ailing flooring retailer Carpetright slipped 3.3% on announcing that sales 'remained negative' amid volatile trading patterns and weak consumer confidence.
Hostel group Safestay rallied 8.9% to 36.5p as it forecast higher annual earnings on the back of a 39% jump in sales.