Many other countries have SLBMs. They're possibly the most reliable "second-strike" weapon. This was a particular concern for the UK as it's very close (relatively) to the USSR; in the event of a nuclear war the US would have had enough warning to scramble bombers / evacuate the president / etc. in a way that the UK wouldn't.

That's one part of the reason the UK doesn't have air-launched nuclear weapons. But really they fell out of favour ever since Gary Powers was shot down; ballistic missiles are inherently a lot harder to intercept than planes. I suspect the reason other countries still have them is mostly residual, and a reflection of the fact that those are the countries that still fly heavy strategic bombers. If you already have your B-52 (or Russian equivalent) fleet carrying heavy bombs and air-launched cruise missiles, it costs relatively little to maintain the capability to put nuclear warheads on those missiles. AFAIK the UK simply doesn't fly a heavy bomber any more - only the Tornado.

Land-based silos make sense for countries with large areas of empty, relatively unpopulated land, which the UK isn't. France doesn't have any either. I suspect India only uses land-based weapons because it can't maintain a sufficiently reliable submarine fleet.