Blue chip shares were in positive territory after economic data showed Britain's powerhouse services sector was in good shape.
The closely-watched Markit/CIPS UK Services Purchasing Managers' Index (PMI) lifted to 55.0 in March from 53.3 the month before, the fastest expansion since December and above economists' expectations of a 53.4 mark.
The FTSE 100 Index lifted 17.3 points to 7339.1 in afternoon trading, as dealers reflected on the figures that are a sharp rebound from February, when the sector reported a five-month low. The FTSE 250 rose 9.8 points to 19030.
IG market analyst Joshua Mahony said: "This morning saw the UK services PMI number provide a pleasant surprise in the wake of the disappointing manufacturing and construction figures earlier this week."
However, shares retreated from highs early in the session as the pound rose 0.3% against the dollar to $1.2476 following the release of the services data.
Top flight shares lose their appeal to investors when the pound rises because most of the sales of firms in the index are earned aboard, hitting revenues when translated back into sterling.
In London's second tier market, oil services company Wood Group announced that cost synergies from its £2.2bn ($2.7bn) merger with rival Amec Foster Wheeler will be 36% higher than previously estimated.
The firm said it now expects cost savings of at least £150m per annum by the end of the third year after the merger goes through, higher than the previous estimate of £110m. Shares in Wood Group lifted 3%, or 23p to 784p.
The biggest afternoon risers in the FTSE 100 Index were BHP Billiton (+32.5p to 1288p), Antofagasta (+20.5p to 860.5p), ITV (+5p to 218.5p), Provident Financial (+59p to 3131p) and DCC (+130p to 7150p).
The biggest afternoon fallers in the FTSE 100 Index were Old Mutual (-7.3p to 191.4p), Croda International (-98p to 3518p), Shire (-94.5p to 4657.5p), AstraZeneca (-81.5p to 4873.5p) and Royal Mail (-5.5p to 411.7p).
The biggest afternoon risers in the FTSE 250 Index were Kaz Minerals (+33.8p to 498p), CLS Holdings (+94p to 1845p), Vedanta Resources (+41.5p to 848.5p), Evraz (+10p to 238.1p) and Hunting (+22p to 579.5p).
The biggest afternoon fallers in the FTSE 250 Index were Allied Minds (-50.1p to 211p), IP Group (-6.8p to 144.2p), Petrofac (-33.5p to 889.5p), JRP Group (-3.8p to 126.1p) and Berkeley Group (-90p to 3065p).