Basically, NATS (The ATC provider at Heathrow) in its' wisdom has decided that the controllers will change from using the printed paper strips to an electronic system, AND from the old tower to the new one at the same time.
Because of possible problems with this, flow rerstrictions wil have had to be applied, probably for a little while (Maybe a couple of weeks) until the controllers are up to speed with the new tower, and the new strips.
All I can say is that air traffic control, particularly at heathrow isn't like workiong in an office, and that, in order to maintain safety, when there are big changes, you can't just swop over;- the number of aeroplanes has to be reduced.
If I know my colleagues at LHR though, things will be back to normal very soon.
Having said that, they bave been given a shonky cobbled together electronic flight progress system that is really rather restrictive, and basically been told to get on with it. Don't worry though, all the major UK airports will have it soon