Normally traffic is to be transferred to the receiving centre in level flight. This normally takes place up to 3 mins from the boundary. And so the traffic should ideally be in level flight at the level in the ACT. Generally people do their best and use the cross LIFFY instruction but unforunately there has been quite a few occasions were the instruction to cross LIFFY hasn't really been followed and aircraft can go sailing through Sector 7 causing some late coordination with London.
The cross 5 or 10 before at the level builds in a bit of protection.