On the blood thing, having recently given my 50th donation[1], I can confirm that (full-blood, and even component) blood donation nets you nothing, in the UK. Except biscuits a drink and the occasional "Thank you" present like table mats, certificates, mugs, badges. None of which are really a solution
I'm probably not the best person to OP on getting extra cash. Something like an eBay blitz might work for some of that (depending on what you can (legitimately!) get together to sell, and with a bit of luck on each item).
Jobs aren't that available around my area for all kinds of people (professionals might get semi-skilled jobs, semi-skilled workers might get low-skill jobs, low-skill workers might get no-skill jobs and the dregs have no hope, along with quite a few that aren't dregs who miss out on (or can't afford to participate in) the 'trickle-down' job-market. I don't know what it's like in your area, but I wish you good luck in getting something yourself that's worthwhile if you are persevering in that.
Make sure that they don't have a problem with you having a holiday in three months (some jobs accumulate a right for so many days of holiday able to be taken in stages as your employment continues, even if you are otherwise already committed for the whole first calendar year which in subsequent years would give you more than enough allocation) and .
I could absolutely not condone anything downright illegal, and would discourage game-of-chance accumulation of wealth. But that's both because I could not foresee myself doing them nor do I know if you have any of the necessary aptitudes to be successful in either dilemma. (Plus, if successful in avoiding the respective pitfalls, do you have either the strength of mind to count yourself on top and keep away from that after your trip, or the courage of your (hopefully not literal) convictions to be at least as cautious in all future ventures of this kind.)
All I have is negative advice, it seems. "This won't work; this'll not be your salvation; you'll be lucky; be good... and, if you can't be good, be careful..."
Fakeedit: Children's book? A good idea, maybe. In fact any book. But even a children's book may take a lot longer than three months to come to fruition (even with an advance). Darn, being negative again. All right, let's reverse that. Still give it a go, if you have the time to dedicate to it. Children's books are shorter, so can be both created and honed quicker (if you have the skill) than adult ones. The concept of an advance means you can at least get something (more than £800..? I assume so) if you can market it to a publishing house, even though it'll take a bit longer to get an artist to illustrate it appropriately, etc, etc... and finally get published... what... six to nine months down the line? I've no idea, figures plucked out of mid-air. But I'd love to know that you managed to do this. Or something similar.
Move onto books for Young Adults, and you're potentially in the area of Harry Potter. Well, we can dream for you... Someone has to be the next JKR. (Although I'd prefer you to be the next Pratchett, from a personal author-preference POV. Minus the health problems, I must stress.)
[1] Could have been a few more, by now, but I've not always kept to their "Come every 16 weeks" invitation schedule, missing some donation periods altogether and delaying others. (And you can even donate slightly more regularly, I think every 12 weeks, but I've only a couple of times thought to wander in prior to an invitation letter to be told that it would be Ok to do so, prior to the holiday or business trip that I was fearing would delay my next donation.)