A somwehat agressive reply... but here's the text from a recent NOTAM (my highlighting).
QUOTE
(H1529/10 NOTAMN
Q) EGXX/QWWXX/IV/NBO/W /000/200/5624N00804W415
A) EGTT EGGX EGPX B) 1005161800 C) 1005162359
E) VOLCANIC ASH IN THE FLW AREA HAS BEEN FORECAST.
6100N 01132W - 5918N 00500W - 5822N 00419W - 5725N 00426W - 5649N 00507W - 5602N 00459W - 5523N 00412W - 5503N 00155W - 5445N 00050W - 5436N 00031W - 5343N 00145E - 5335N 00159E - 5306N 00222E - 5244N 00151E - 5230N 00119E - 5227N 00040E - 5210N 00001E - 5155N 00127W - 5219N 00244W - 5251N 00308W - 5305N 00340W - 5217N 00624W - 5207N 00750W - 5207N 00810W - 5446N 01520W - 5607N 01858W - 5633N 01902W - 6100N 01735W - 6100N 01132W THE AREA IS
DESIGNATED A NO FLY ZONE (NFZ)(ICAO DEFINTION: AREAS OF HIGH CONTAMINATION) SINCE THE CONCENTRATION OF ASH MAY EXCEED TEN TIMES THE STANDARD THRESHOLD AND THEREFORE MAY EXCEED SAFE LIMITS.
NO CLEARANCES WILL BE ISSUED TO PENETRATE THE NFZ. NOTAM FOR ADJACENT FIR SHOULD BE EXAMINED TO ESTABLISH FURTHER NFZ. IN ADDN THERE ARE VOLUMES OF AIRSPACE WHERE ADDITIONAL PROCEDURES MAY APPLY (ENHANCED PROCEDURE ZONES (EPZ) (ICAO DEFINITION: AREAS OF LOW CONTAMINATION).
UK ACFT OPR CERTIFICATE (AOC) HOLDERS PLANNING TO OPR WI AN EPZ SHOULD COMPLY WITH THE LATEST FLT OPS DIVISION COM (FODCOM) DOCUMENTS, AVBL AT
The UK Civil Aviation Authority (SEARCH FOR: FODCOMS PUBLICATIONS AND FODCOMS FOR GENERAL AVIATION). ALL NON-UK AOC HOLDERS PLANNING TO OPR WI THE EPZ SHOULD SATISFY THEMSELVES THAT THEY ARE ABLE TO OPERATE SAFELY. THE DIMENSIONS OF THE EPZ ARE AVBL FM THE VAAC, AVBL AT
WWW.METOFFICE.GOV.UK/AVIATION/VAAC (FOLLOW THE LONDON VAA-ISSUED GRAPHICS LINK). NOTE - THE FORECAST DIMENSIONS OF BOTH THE NFZ AND EPZ ARE UPDATED EVERY 6 HR.
F) SFC G) FL200)
I also hear a rumour that someone who flew VFR in a No Fly Zone the last time around is being investigated with a view to prosecution. I'm not a lawyer, but that suggests to me that there is some statute which makes this possible.