Almost certainly would be cheaper - Sunday evening from London are probably the most expensive flights of the week. The OP would have had hotel charges and missed work though.What one could do is get on to the web and book another flight for the following day if necessary.....bound to be cheaper than £200.
The problem with that is you would get people trying to work the system to get cheaper flights (human nature) - Sunday evening flights are expensive so why not book an early morning cheaper one, 'miss' it and then ask to be let on cheaply to the later flight...While I don't believe that passengers who misread tickets should get another flight free of charge it is a bit severe to charge what the contributor above was charged.
There obviously was capacity on the plane which they could use. Impose some penalty but no need to crucify passengers
In fairness to AL, they do give the best available fare to people who miss flights (have done this a couple of times myself and got my husband to check on line to see if he could get it cheaper) and a big part of the OP's problem was the day and time she was travelling - if it was a Wednesday morning, it might have cost £75 but a last minute Sunday evening fare from London will always be expensive.