PS, the crunch date is not the cut off for applications, but the date of offer and the date of starting school. If you are offered and accept a place in March or whenever it is then it is your place as long as you have been and continue to be honest about where you live at all points throughout the process, including before your child starts school.
However, given that your move may take a while, you might want to look at this in the admissions brochure:
If you move at any time during the admissions process (the admissions process covers the entire period from submitting your application to your child starting school), you must provide evidence to support the reason for your move.
If you move to a new address without telling us and secure an offer, based on your old address, of a school place that you would not have been offered based on the new address, we will treat this as misleading information and the offer of the school place will be withdrawn.
If you move to a new address and have advised us, once our address verification procedures have been satisfied, we will revise your child’s position on the waiting lists for each of the schools that you prefer so that you receive the correct outcome.
If such a move takes place after you have received an offer of a school place, in order to be permitted to retain the offer, you must satisfy the Council that the change of address was not anticipated prior to the offer of a school place.
In your situation if you have a good reason for the move (eg you are expecting another child and need more space, your existing child is getting older and you currently live in a flat and would like a garden, you need more space for an aged parent to come and live with you, you got a dog/cat/rabbit and need a garden, you can't afford your current place and need to downsize) they won't think you are gaming the system. However as you are moving further away from the school you want, you will probably have to show that you have a good reason for this and make sure to notify them on time about your move. In addition, if you offer on the house and it's accepted and you think it will go through and you will end up living there, I would tell them sooner rather than later. When people do game the system round here, they do exactly what you are doing - get into preferred school and then move away to an area near a less popular school. Also, your move is very much anticipated. So, in your shoes, I would call the admissions team and explain what is going on and ask them what you should do in order not to have your application treated as fraudulent. They are very nice, both on email and on the phone and will help.
Having said all that, as you think your new address will be OK for the school you want, it should not make too much difference. Best, though, to communicate with the admissions team in advance so they can see you are on the level.