Again I can't speak for the UK but here in the Middle East and particularly into Dubai, just like Scott said, if you are going to fly economical speeds then tell us on first contact. If you surprise us then we will probably surprise you back, ie. you will be moved back in the sequence. Do us and yourselves a favour and advise the ATC'ers of your intended speed if it is something unexpected for the aircraft you are flying (for a modern jet anything from 270 kts or less). In this part of the world at least the usual speed we expect is 280kts and upwards until STAR speed restrictions take effect. If you give us less than that without asking and we don't have time to have a friendly chat with you about your speed, you may find that that economical speed won't be so economical as you get given a radar vectored tour of the Gulf or in some cases, you can fly an even more economical speed going round in the hold.
Bottom line, let us know early so we can either plan to accomodate you, or tell you no and make you speed up.