Also thanks for the link. I hadn't thought of it like that.
From my perspective, £18k (and indeed £28k) is a lot of money. I'm 19 and I'm getting paid to code, I know it's naive but I think it's awesome. I love the people I work for/with and I love my job. I come across adults who haven't had that their entire career.
Edit: Just to follow up on the counter offer. The process of raising this round of funding has been such a whirlwind. I don't think there is anything untoward going on. I think a case of Hanlon's Razor: http://en.wikipedia.org/wiki/Hanlon%27s_razor
But 18k doesn't sound far off the salary I'd expect an inexperienced junior dev to make. Things that might get you a higher salary for this kind of role are a degree or prior experience. If you have prior working experience (even freelance) then you probably are getting a little too low.
Around 26k would be nearer the mark for a starting grad with no experience.
http://corcodilos.com/blog/7874/should-i-take-a-big-counter-...
The job offers you got suggest that your current employer has been happily underpaying you by 50% of your market value.
Also, you're comparing a "hard offer of £28k" with "informally agreed to better the £28k offer". Don't turn down the hard offer until you've gotten a real commitment. And you'd need to get that commitment before the £28k offer expires.