Offer handyman services - painting, fixing a garden fence, etc. If you do a good job, you'll be inundated with work.
You could earn your £300 in just a weekend.
+1
Where I live getting someone to do a few simple handyman jobs is like your asking for the holy grail. I stuck a post on Facebook and said I would pay £15 per hour cash for painting a fence, I got loads of abuse saying I was underpaying and £25 per hour was a fair wage.
I pay £25 per hour for a gardener, he has all of his own equipment. I need him for at least four hours per week, but he will only do two - because he is so busy.
Get some simple, but classy leaflets printed with what you can / will do, your rates and a little background story. Most importantly,
find a nice neighbourhood and post through their letterboxes and knock if you're man enough.
That's how my gardener got started, now he employs two people full time and has a smart sign written van.
You'll obviously need to have some gardening tools, or invest in some.
If you want to sit at home and earn, then you'll need to have or develop an in demand skill - there's tons of people sitting on their backside thinking that money is going to drop in their lap, or they're going to make money from Bitcoin or some other get rich quick scheme. The only people making money from those schemes are the ones selling you courses on how to do it.
If you want something, you have to go and get it.