I am not in the UK but there are lots of people on this site who are. I don't remember any one of them speaking like this.
(Ok this is serious advice but do whatever you want with it)
Go get a job at McDonald's on the days your not with the kids and use every penny (or whatever currency is called) to hire a lawyer.
Do NOT speak to your wife about it, just go do it. Then have your lawyer draft a letter to your wife stating what days you will have the kids and if there are any disagreements you will be happy to peruse legal action, including kidnapping charges.
Me: 35 husband:39 Sons 16 and 11 from my first marriage Twins 5 (boy/girl) Daughter 3 Affair bomb 2/27/14 He moved in with ow 3/13/14 OW kicked him out 6/15/14 4/2016 he seeks help for sexual addiction