they said because of the work to rule we would have to file a "proper" plan.
As in file it with Flight Plan Reception (assuming Dublin has one) rather than locally? It all depends on whether it was actually part of their job to file plans for you or just something they did as a courtesy. With commercial airlines, they have an Ops dept to do that sort of thing and it's not usually up to the local ATCU to do it. Was there a local agreement for the non-com operations that ATC would file all plans? In my experience, this has been done as a favour to non-com operators rather than a right that they can expect. I am speaking from a UK based perspective though.
Look at it another way, if there is something you do that helps the operation but isn't strictly part of your job, and your employer insists that you do it despite not sticking to what it has agreed to do, the first place you would look to cut back on in a dispute would be those non-obligatory tasks, is it not?
Anyhoo,
EW and co are still hard at work up at Muckamore Intl and I'm sure they would be willing to get you out to East Mids or Birmingham on time. It's only a couple of hours up the island by road.
That said, Up The Workers, make an Impact!