gotravel insurance is who we always use, but i've phoned a few places as i'm asthmatic too and the general rule is as long as you haven't been hospitalised within the last year because of asthma related illness you won't need any extra cover
I opened a Halifax Reward Account for the insurance, it costs £10 per month for Multi Annual cover for all the family, Also loads of other Benefits like RAC and Mobile Phone cover included, Too much to list.
Great if you travel more than once a year.
Its on the halifax.co.uk website.