Hi Tricia - I'm from the UK and going on holiday in April too! I'm just going to look on MoneySupermarket to see about insurance - it's the easiest place to get a quote without having to commit. If I find anything especially portentous I'll let you know!
update! I just got some for £15 (single trip). MoneySupermarket gave me the option of declaring my ileostomy, turns out it didn't even add anything to the insurance, so that's good to know. It's worth having just for a bit of peace of mind, especially as the idea of medical bills is so alien to an NHS-er!