Second line UK books are not much safer than most offshore books. In order of safety my top 5 would be:
hills - biggest market cap book in world I believe, as safe as a second line bank with a very strong balance sheet
ladbrokes - strong book, not as solid as Hills due to the hilton hotels side but their bonds would still get at least an A rating from moody's I'd think.
corals - very solid
paddys - quoted with a strong balance sheet, could challenge the big 3
chandler - not quoted, but clearly solid. Has a lot of high rollers
stan james - pretty solid, high rollers.
Overall though Betfair must be right up there as they have such a high volume on their exchange and have a very solid business model in my view.