When to unenroll in the US and enroll in the UK High School?

Soon will be relocating to the UK with my daughter that is age 14.  She is enrolled in ECOT, which is a home school program in the US. 
After we relocate to the UK, just curious when do I enroll her in the UK schools?  Prior to leaving or after living in the UK.  Is there a time frame this has to be done?

Thank you for any advice.

I believe that you will not be able to enroll your daughter until you have proof of an address (tenancy agreement or exchange of contract) here in the UK.  Also, keep in mind that the school in your catchment area may be oversubscribed. 
If you know where in the UK you are moving to you may want to look at the council's website. 

time frame wise I'd do it ASAP as soon as you get here. Like Red5 said, if you know where you will be living contact the council.

Thank you both.  I will be in St. Anns, Nottingham, so, will contact the Council there.  Thank you both so much for your advice.