The decisions are being made based on the Ash charts that are published every 6 hours by the met office.
The same Ash Charts that were used when Scottish and Northern English airspace was 'opened' on an opportune basis as the charts indicated was viable over the past 5 days.
Of course the cynics on here who believe in conspiracy theories etc will try to claim that the Met office is being forced into doctoring the charts because of so called 'test' flights by the likes of BA.
I'd rather believe the charts than take data from one flight by BA as being an indication that is is 100% safe.
This whole episode is reliant on science - one flight over UK airspace is not sound basis for decision making and has nothing to do with airspace becoming available to IFR traffic tomorrow.