Unless you already have top of market compensation, I'd focus on interviewing for a higher paying job rather than do more work on the side. Going by the ££ signs I'm assuming you can get a job in London, where you can make >£200k total compensation as a good software engineer.
I'm already earning near that compensation, was just looking for something on the side as well :)
If you're just talking side gigs be really clear this doesn't interfere with your own job performance. The first target you hit because of your "side gig" will be your downward spiral.
Now .. running your own business is another option. For me I encourage my staff to have "side gig" of their own business, but will not sign off on them contracting on the side.
One is a valuable option where you learn new skills - the other is...