LONDON — A pandemic forcing everyone to stay home could be the perfect moment for online grocery services. In practice, they’ve been struggling to keep up with a surge in orders, highlighting their limited ability to respond to an unprecedented onslaught of demand. After panic buying left store shelves stripped of staples like pasta, canned goods and toilet paper, many shoppers quickly found online grocery delivery slots almost impossible to come by, too. “It’s kind of becoming more challenging to put a meal together,” said Paul Smyth, a software engineer who lives near Manchester, England, where the online groceries industry is particularly advanced.