£18,000 for a PPL?
£12,000 for a PPL?
Which ever figure's correct that's way too much.
The difference in gas prices accounts for some but the biggest problem from what I've seen is instructors charge way too much in the UK. I've seen figures between £25 and £40 an hour quoted in this forum.
When doing the math you'll have to allow for flights, housing, checkrides with an instructor when you get home but I reckon you're better off training in a country where gas is cheaper and instructors teaching PPL don't charge so much.