Surely your rate needs to reflect where you are and how much others are charging?
ÂŁ20 is nonsense either way. People call a spark as they want to sleep safe at night knowing the work was carried out by a pro, so being that cheap is as likely to lose business as win it. And then when you do win it you're hardly making any money.
I suppose the first move is to check out the rates of other local sparks. If you want to be busy in a hurry, go for a mall but worthwhile undercut on their rates until you're established in the area.
At least the feedback you're getting is positive though, you're about to start earning way more than you apparently thought you would