In the dim and distant past when I went to shows in London, I always enjoyed an italian meal at the Paradiso e Inferno on the Strand, just a short walk down from Covent Garden. I've checked and it still exists today http://www.paradisoeinferno.co.uk/. If I remember correctly they are also open late into the evening so you can dine after the show ... I'd strongly recommend reserving in advance because it was always busy. I'm not sure if the prices are reasonable for London, but I remember the portions being generous. There are plenty of pasta dishes and starters too, but they aren't shown on the website.