The defence is easy. ICAO documentation, of which the UK is a signatory, paraphrasing said 'Volcanic ash then no IFR flights'. The UK had not filed a difference with ICAO and therefore was bound by it, regardless of whether it was the best option or not. There are IMHO better options out there (Alaskan procedures look a great starting point), which in slower time could be developed and agreed as international policy.
I couldn't find anywhere in the ICAO document where is said if there is
any volcanic ash then no IFR. If it did there would be no IFR flight ever as there is always at least some background level of ash. Doc 19 just talks about ash clouds but doesn't define what level consitutes a cloud (interestingly, I don't think anyone defines what specific nature of condensing liquid is a weather cloud either - maybe just judgement is used on this
). Doc 9691 also discusses ash clouds and specifically says
The question at issue is — when does the concentration of ash in the contaminated airspace decrease
to a level considered safe for aircraft? Moreover, flying through even very low ash concentrations considered safe from the
standpoint of immediate engine damage may, as indicated in 2.1.4, still cause long-term engine damage, with significant
economic consequences. These questions are discussed in more detail in Chapter 4.
Based on the incidents documented in 9691, many other aviation authorities seem to have choosen a much more restricted limitation of flights and to use Cloud to mean something appearing like a cloud (rather than a probabilisitic distribution of where ash might be as used in Europe).
As such, I think the argument 'ICAO said, we signed, therefore we did' is a bit shallow.
The decision may or may not have been the right one, but hopefully it was made on something other than Doc 19 requires no IFR if the metoffice says 'ash may be thar!'