If you're not waiting for a promotion or anything I think you should tell your employer soon because you will have a little bit more protection and your employer will have more time to find someone to cover you.
You don't have to tell your employer until 15 weeks before the beginning of the week of your due date even with all of the issues you discuss. However if you want your boss to be aware/sympathetic about your health when you return you should tell sooner. Note that unless your employer has an enhanced policy you are only entitled to parental leave if you have been employed for over a year so if your career break broke your employment (which i assume it did otherwise you would be entitled to SMP) they don't have to agree to let you take it. Also it's only 4 weeks per child per year .. is this why you mention the new tax year?
Well you don't have to tell them right now, but given you are only going to be back at work for a very short time, it would be considerate to tell them as soon as possible, yes.
I think the nice thing to do would be to ring your manager for a catch up, and mention it then, then follow up with an email confirming it. Or even better, meet up with your boss for a coffee or something to catch up and discuss the forthcoming year.