If you can afford it, pay it imho. You said you'd buy a wedding cake and like it or not £450 for a wedding cake is pretty reasonable by a lot of Bridezillas standards. I personally think wedding cakes are a huge amount for what they are too but unless you cant afford it you did say you'd buy a cake without really clarifying how much that would caost you.
If you c ant aford it, I'd go with something like
' Dear Mork and Mindy, I'm so excited about your wedding and also about being able to contribute to your wedding cake. Here is a cheque for £250 towards your cake, which I'm sure will be amazing, I cant wait to see it. With love and best wishes, Auntie Retrochic xxx