Wilkinsons have good plain white cases. Sainsburys have pretty and cheap cases.
But do a trial run first with your chosen cupcakes, so you can check that the butter doesn't leak through or that the cases peel away from the cupcakes.
You can't freeze the buttercream on the cupcake, but you could freeze the cupcakes well wrapped/sealed. Let them defrost properly and then decorate. You can make the buttercream a few days beforehand, and then bring it to room temperature, whip it up again and you're ready to go.
If you are putting fresh fruit on, you need to do that at the least minute, as the juices do leak out after a while.
You could make some loveheart decorations very cheaply. Get some block fondant icing from a supermarket, costs about £1. Some red gel food colouring. Get mixing. Roll out thinly (about thickness of £1 coin) and with a heart cutter make loads of hearts! Leave them out to harden over 24-48 hours. Store them somewhere without a sealed lid, eg a cardboard box, not tupperware as they will "melt" and soften. These you could get done and out of the way well before August.
The Hummingbird chocolate recipe is good, it's on the Tesco website.
You can't go wrong with a lovely plain vanilla one, just make sure you buy vanilla extract and not the essence, as the essence tastes really synthetic.
I have offered to make cupcakes for an engagement party in August. I think I will make 50 vanilla and 50 chocolate, bake ahead and freeze. The buttercream can also be frozen can't it? What I'm after is some good (non-extravagant) recipes that are nice and easy. I am a confident baker so not too worried. I was thinking of having a look at some of the Hummingbird Bakery recipes. Also any tips for where to buy nice but not too expensive cupcake cases? I was wondering about Wilkinsons. And I would also love any ideas for great decorations that won't cost the earth. I did wonder about strawberries for the vanilla ones. I guess the theme is nice but cheap!!