Progressing from AAT to ACA - a few questions

Hi,

I'm currently studying AAT Level 3 with a local College in Portsmouth, whilst working in a private practice that specialises in tax.

I want to advance on to ACA afterwards, I have a few queries, mainly focussed on the AAT - ACA transition.

Firstly how much credit do ICAEW actually give you for having completed AAT? I've read a bit about being able to get most of the work experience part of the ACA exempted? By registering with an ICAEW training employer provider (or something along those lines) and being able to discount up to 12 months if you're working while doing level 4.

I don't think my current employer is ICAEW registered (nor am I sure they'd be able to, not sure how easy this is or not) so would I need to find a new job for my final AAT year with someone who is who can then do the training agreement with?

Then i'm wondering when I have the AAT and am ready to move on to ACA, who can I apply to? Do any of the Big4 offer AAT-ACA fast track or something equivalent?? I checked out PWC but nothing jumped out at me. I think getting ACA with one of the Big4 is probably my aim.

After completing AAT, assuming I get the 12 month discount from the 450 work days, how long does it take to do the ACA? I assume this varies slightly from place to place?

Thanks for your time guys.