I think with the suggestion regarding a share of your return, you'd be looking at an investor, not a sponsor. But the kind of money you're asking (£300), it's not the kind of sum that would make it worth while most potential investors doing any due dilligence.
I think straight forward sponsorship would be better. I like the suggestion of working iwth a bridal shop or similar.
As for letting them know - phone/write/visit the potential sponsors. How many are you looking for? 5? a dozen, hundreds? I'm asssuming only a low amount, so direct contact would probably be the best approach.