In terms of price Discount TV seem to be the cheapest atm (£1235), but they don't include a screen anymore or appear to do finance if that's important to you. AV-Sales include a screen but are more expensive at £1325 and again I don't think they do finance. Robert Whyte do finance (currently a pay nothing till 2005 deal) but don't list the AE500, you could give them a ring and see when they'll get some in, what price they'd be.
You could always get a new credit card with a nice long 0% Balance Transfer deal, buy the AE500 on your existing card then do a balance transfer to the new one. You'd then have 0% for 6/9 months or whatever, it depends how long you were planning on taking to repay.
But as with all credit, make sure you can actually afford it. No point being too distracted by whether you'll make the repayments to enjoy your home cinema.
get a capital one card where you can have 0% for since mths and 3.9% apr for balance transfers for life, what i did was put my pj on my old card then transferred the balance to my new capital one card and now i am only paying an apr of 3.9% for the pj till iv paid for it.
I would suggest (although the idea disgusts me on many levels) that you actually purchase your projector with Barclaycard and pay it off as quick as possible.
The price promise that they run will probably save you a lot of money given the way projectors drop in price. Plus, it will allow you to buy the projector from the best and most reliable dealer now, and then price match against the cheapest box shifter you can find in 60 days time.