I think 20% is more than a 'marginal difference in price'; ask anyone in the UK if they would think a price reduction of 20 pence per litre would be worthwhile!
Don't forget there are large areas of the UK without public transport; Scotland alone is 4 times larger than Cyprus and much of Scotland is without public transport - and public transport here (buses, service taxis etc) is much cheaper than the UK. One stop on the London 'underground' cost £4 me last time I was there!
I will stick with my cheap Cyprus petrol, even if the thin hot air and blasting a/c in the car means I do not get many kilometres to the litre!
Lana