The UK has always lagged well behind America in dev pay and has only been accelerating in pay for the last 5 years. 5 years ago a senior dev position outside London was around 35k. Now it's up to 45k and I'm starting to get emails with 50ks mentioned.
To make '20k' you would have to earn £900 per day, when the going rate outside London is £300 p/d, and even as recently as a year ago it was £250 p/d.
You earn £900 p/d consulting, not developing.
I've always blamed the 'BBC micro' effect for our lower pay, we had a lot more exposure to computers as kids in the 80s and so reached 'peak developer' a lot later than the Americans did. Also in the UK the idea that the peons could be worth more than the managers was anathema until the last decade, a class divide throwback.