Well done.
Don't take any notice of the smart alecs and snide comment merchants on here.
What you will probably find is you will fill your diary, and then increase your prices later.
I'm pretty confident that when you have done a couple of years you'll look at some of the prices you charged at first and think 'That wasn't enough' but hey it's a learning curve.
If you don't charge enough you will chase your own tail and not make much money, but it takes time and in the meantime you need to pick up work.
Keep knocking you'll get the work.
£20 is better than a lot of jobs, but you need to make a decent amount to cover you when things go wrong, or you get ill or the weather is crap for weeks.
All the best