Assuming the volcanic ash is a problem at FL300+, what is dangerous about flight at, say FL250? By all means adjust the levels but a buffer beneath the known levels of volcanic ash (and if 5,000 ft isn't enough, go for 10,000 ft!) would seem a much more proportionate response.
Transatlantic operators probably don't want to to bimble around at FL250 or below, and possibly can't anyway due to fuel burn, but domestic and European flights would be able to operate and you wouldn't be in the slightly ridiculous situation of a large section of the UK's economy grinding to a halt.
Ahh, but then NATS directors probably wouldn't be interviewed on TV and radio and wouldn't be able to bless us with their "wisdom"...