You are of course right that accurate delay info is usefil to have and necessary once delays build up and when I am on the radar I will always try and have a look at the stack on radar and estimate the delay. If traffic permits, I will offer you the chance to slow down en route to save fuel. All the comments so far about the updating of the EAT monitor are valid.
However, the official line that anything up to 20 mins holding is classed as no delay for Heathrow. That's why you may get some ATCOs who you ask for the delay looking at the EAT monitor and answering "Less than 20 mins" when it could be anything from zero to 19 mins. Not awfully helpful, but in line with the procedures!
Given all this, "we have 10 mins holding capability with LGW/STN/LUT options - any more and we either commit or divert." would seem a little bit tight on the fuel planning..