I'm so sorry. This has happened to me in the past.
You need to post on the legal board.
In short both countries are signatories to the Hague Convention and (from what you describe) the children's "place of habitual residence" is Germany. This means if you remove them from Germany without their father's permission he is entitled to file for child abduction against you and for the children to be returned to Germany.
If he gives you permission to take the children out of Germany for a holiday and you do not return, he will also be entitled to file for child abduction.
Your best bet is to get his permission - I know how nightmarish this is for you. I have been there - you are separated from your support and the person whom you thought had your back has disappeared. You are going to have to think with your head and do your grieving later.
If you do not get his permission to relocate with the children, you can apply to the German courts for "leave to remove". Basically, you will need to argue your case of why it would be in the children's best interests for them to move to the UK with you (eg, you are their primary carer). You will have a better case of getting this if you can prove you can provide financially for them in the UK (eg, line up a job); that you will bend over backwards to facilitate contact between the children and their father etc.
Look at www.reunite.org.uk which has a list of UK lawyers experienced in this field. Some will provide a free first consultation. I "shopped around" for mine (ie wrote several an email outlining my circumstances asking what help they could provide and what their fees were).
Good luck. I am thinking of you. Keep strong - you will get there.
xxxxxxx