Well the answer is "they wouldn't".

This route is thoroughly booked up on those dates, the first week of the British Easter holidays and so lots of people want to fly out of London that day. The fare you've mentioned is actually booking into 'Y' class, the most expensive, which would give you full flexibility, but that is rather an enormous price for economy, I agree.

Likewise First class is actually coming up cheaper than Business class.

It's a popular route and gets booked up early!