I don't think they're especially out of order approaching you in the first instance, although telling you how much they want upfront (unless it was a small amount e.g. if they were asking for £10 or £20 from lots of places) is cheeky.
Also, you don't say what kind of social enterprise it is, but it sounds like they should be setting their sights more on actual businesses that are run purely for profit. I work for a small charity and we recently received a letter from another entirely unconnected charity asking us for £1,000! I don't know where they got our name from, but why on earth you'd approach other charities to donate to you, I've no idea.
The letter was written to be quite persuasive, constantly mentioning our charity by name in the blurb alongside the £1,000 that they were after - obviously just an automatic 'find and replace' job, but still quite passive-aggressively styled to make it sound like it was the least that we could do rather than them asking a very big and cheeky favour.
I think the worst thing, from your point of view, is the relentless nature of it. People tend to assume that they're quite the first to think of asking you for money or freebies, making very unoriginal 'jokes' about your name ("Mrs Constable, eh? And are you a police officer? Ho ho ho!!!"), telling you that you're very short or tall etc.
They might not realise that they are just one of many, but combined, they're an immense nuisance - like you don't really care if you see an ant, but thousands of them in your kitchen are really upsetting.
If you bother replying - and you're probably wasting your precious time, as they likely just scattergun them like junk-mailers and ignore any 'negative' or non-responses - I'd point out that you work very hard because you believe passionately in the cause (yours, not theirs!) and therefore, as you already subsidise it heavily yourself, there most certainly is no excess to be giving away to other causes.