Four years ago my family and my friends family went on holiday to Florida. Both our family and my friends family were covered by their bank accounts. My friends youngest son got an ear infection. They rang their bank and went to the hospital, but the hospital wouldn't recognise the details given by the bank, so they ended up paying by credit card. It was almost $100 for the prescription alone, plus the cost of using the hospital. When they got home they received a bill for the Doctors time. It cost over $1000 in total. They did manage to claim it back from their bank, but it took them almost a year.
Ever since then I have always taken out the Post Office travel insurance, as it has won an award for the best travel insurance for I think the past five years. Peace of mind is worth an awful lot! Also, unless you are going to use all the facilities of the bank account, then it isn't really going to be cost effective to pay out £12 a month.