Here I come banging the same old drum, dad.
PLEASE PLEASE PLEASE book an appointment with a solicitor who specialises in divorce and family law.
You were advised to do this when you first posted some 6 weeks ago and I am getting increasingly concerned that, if you don't consult a solicitor soon, you'll miss out on your current legal aid entitlement as this benefit won't be available to you, or anyone else for that matter, after the end of March.
You don't necessarily need to petition to divorce immediately but you do need to secure residency for your boys, and also get the tenancy of your home transferred into your sole name free and clear of rent arrears so that you can apply to move to another property asap if that is what you feel will be in the best interests of your dc.
In addition if, by chance, your w came to be in possession of sufficient funds to launch a custody battle from Greece after the end of March and you have not secured your boys future in the UK you will, to put it mildly, be stuffed as the cost of defending any such proceedings will be beyond your means and you'll be reduced to representing yourself against what may be a formidable legal team acting for your w.
Further, your w's selfishness has been profoundly unsettling for your dc and they need the security of knowing that, although their dm has effectively abandoned them their df is not only there for them now, he's also making sure that he's always going to be there them by taking the necessary legal steps to ensure they can't be taken from him.
Don't miss the boat, dad. The forthcoming changes to legal aid entitlement are causing solciitors' diaries to become fully booked through March and the sooner you make an appointment the better.