The restrictions are less to do with immigration/passports and more to do with Customs. I think that B747-437B has explained elsewhere that there was a lot of revenue fraud being committed by passengers who went into the controlled area (airside international departures) and then voluntarily abandoned their flights and went landside again. It's not difficult to see how much money can be fraudulently made this way, nor why HMRC is so keen to make it difficult to do it.