I've taken my first wedding cake order (very excited) and have of course asked for a 50% deposit. Having read various websites it seems everyone asks for the remaining amount 2-3 weeks prior to delivery, is this correct? I'm in the UK. (My general celebration cakes I get the final installment upon delivery) 

Yeppers, although I know some require the final payment a month before the wedding (I think K8 said she does).  And don't start working on anything (or buy anything) until you do have that final payment.

Always get it at least three weeks before,

I'd say check around and see what others do in your area.  That said, I usually ask for final payment a week in advance.  It's too much hassle on wedding day for the bride or whomever is paying to deal with money.

I ask for final payment 2 weeks before, some couples take a week to actually proceed with a payment, that's why I would worry to ask for a balance a week in before their wedding. 

Under NO circumstances should you wait to get paid until you deliver the cake.

My contract says 30 days before the reception.  You should get it no later than 2- 3 weeks before the wedding and DO NOT make anything until you get the final payment.

50% retainer and final payment five weeks ahead.

Great thank you all for the advice