EDDNR
I assume when you say no holding fuel for LHR is carried that you mean that over and above that which the company specifies as part of the minimum reserves. I sure hope so
The UK uses the term 'No Delay Expected' as a standard phrase. This doesn't mean that you won't be held, it means that you can expect holding to be less than 20 minutes before you will be permitted to commence an approach. The controllers can give an indication of the likely holding time on request.
When delays are expected to be 20 minutes or more, you will be issued with an Expected Appproach Time (EAT).
The UK CAA recommend that these factors are taken into account when planning trip fuel. They also recommend that commanders who expect to arrive with little more than their company minimums establish the weather conditions at destination and alternate plus the delay status. As stated by Digitalis above, that then gives the commander the chance to make an early decision on his intentions without a mad panic at the end. Of course, the unexpected can always happen which changes everything (exactly what EDDNR stated happened), however plans can only be made on the best information available at the time. ATC are always a flexible part of the operation, it's our job. Just let us know as SOON as you can. None of us wants to see something like the Avianca crash in NYC happen here in the UK, trust me.
Further information is in the UK Aeronautical Information Circular 36/1998 (Pink 170)
You can read it
here though you will have to register for the AIS site if you haven't already.
Cheers