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!