Oil prices also lifted, with Brent crude more than 1% up at 48.03 US dollars a barrel.
Tony Cross, market analyst at Trustnet Direct, said while the UK’s credit downgrade was a blow, it meant “just a little of the uncertainty is starting to ebb away”.
“We may not be out of the storm, but hopefully the worst is behind us – the market does seem to be trying to find a level,” he added.
Airlines were also among stocks surging higher, pulling out of their nosedive sparked off by the Brexit vote.
British Airways owner International Consolidated Airlines and budget carrier easyJet were up 4% each.
Rolls-Royce was another enjoying gains – up 2% – as it reaffirmed its commitment to the UK despite Brexit and said currency movements would boost underlying revenues by around £400 million and improve underlying profit before tax by around £40 million.