You are not expected to climb with flaps extended...on a fully loaded B744 out of LHR min flap-up speed is around 263 kts and a margin of at least 20 kts is added to that. If you advise ATC of the minimum speed you require you will not be held at 250kts. On descent, approaching normal landing weights, 250kts is not a problem. What can be a problem is the UK max holding speed at lower levels of 220kts...although again requests for higher speeds are normally always given.