From NHS website...
"Your EHIC will allow you access to the same state-provided healthcare as a resident of the country you are visiting. However, many countries expect the patient to pay towards their treatment, and even with an EHIC, you might be expected to do the same. You may be able to seek reimbursement for this cost when you are back in the UK if you are not able to do so in the other country.
The EHIC is NOT an alternative to travel insurance. It will not cover any private medical healthcare or the cost of things such as mountain rescue in ski resorts, repatriation to the UK or lost or stolen property. "
I just went for a week's holiday to the Canaries and took out travel insurance for first time since diagnosis. If you're going to take out insurance, make sure you get a policy where you can declare your condition, even if you decide not to cover it. A lot of the cheaper policies on the comparison websites doon't let you declare pre-existing conditions and if they find out about it, it can invalidate your whole policy...even for something totally unrelated to the Crohn's.