It is an advisory instruction only.
If as an ATCO you want to issue speeds greater than 270kts, you can do so. As pointed out, you need more flexibility to stream aircraft.
Having everything at 270Kts could actually cause holding in situations where normally the use of high speed on the first couple means everything comes straight off.
how will the sectors know there will be no holding? CLN, for example are working traffic 20 minutes from LAM, how do they know what traffic is pending for OCK, BIG, BNN?
The 'habit' very quickly will become 270kts for everything. There's no way Heathrow will phone TC NE to tell TC EAST to tell CLN to keep the first few high speed (that's assuming LL identify a gap in the traffic 20 minutes away!)
At best you'll get 270kts from CLN, 270kts from EAST then 320kts from NE/LAM which is a waste of fuel along the lines of BOAC's post.