Georgieboy
As medical treatment is so expensive in America it would not be unusual for someone to incur costs in excess of £100,000 as a result of a heart condition. As you have previously had a heart attack the standard cost of insurance is increased to take into account the extra risk. Whilst £800 would not come anywhere near covering that cost insurers do quote additional premiums in the hope that there will be sufficient money in the pool to cover loss of this size.
This type of premium is not a rip off, it's just a reflection of the additional risk that your personal circumstances bring to the pool.
Hope this helps