Okay, please excuse my ignorance if there's something I'm missing here.
I understand the problem when it comes to saving, and the issue of dipping into savings when you shouldn't. But I really don't see the point in a service like this, at least not personally.
There's three main issues I have with it:
1. Why should I let some random company I don't know or trust keep my savings for me? I would assume you're not a actual bank, so should you go bankrupt there's no government insurance that'd ensure I don't loose my money. And not saying you would, but from my point of view, what's there to stop you from taking the money and run?
2. A charge of 3%. So to save £100 I'd loose £3. If I was saving for a Retina MBP of around £2,500 I'd lose £75.
3. Interest. Ok, sure, banks pay pretty horrible interest, but still, I'm loosing out of money I would otherwise have gotten. I saw your response about working on providing interest somehow in the future, but I'm focusing on the now here :)
When you add it all up, the increased risk of losing all the savings, combined with the loss of money in the 3% charge and lack of interest, personally I really see no point in using your service compared to my bank's savings account.
And ok, it's easy to move money out of my savings account, but it's supposedly easy moving it out your service too. If you were to change that, there might just possibly be some point to it compared to a savings account. Except then you're treading on dangerous legal and/or moral ground if you deny people access to their money, even if they've previously agreed to the terms.
On a final note, I believe the only valuable feature you have to offer is the fact that friends can chip in. If I were you, I'd focus on that feature instead of holding people's cash. Let people set up some kinda campaign page for their new phone/laptop purchase and let family and friends chip in, wiring the money back to the user, rather than you holding on to it. Think of it like a personalized kickstarter mashed with amazon wishlists or something.
Let little Jimmy Neutron setup a campaign for his wish to get an Xbox One. He's got £200 cash already, which the campaign reflects, then he sends the campaign page to his grand parents, uncles, aunts, etc, and maybe they'll chip in enough so he can get his new console.