Jersey is a good place, 3 weeks is maybe pushing your luck but 4 weeks is almost guaranteed.
Did mine there in 3 weeks 2 days (including 1 day off a week).
All training is over water, so you don't develop a fear of it.
The airport is busy, with commercial props & jets, so you become used to fast reaction times & crisp busy Radio.
All cross countries are over France, remember your passport. You also learn international procedures, don't get that as part of a mainland PPL.
Food is good & drink, relatively cheap.
Instructors & staff at the club are very nice & friendly, they will try their hardest to get you to a pass before you leave the island.
The overall package gives you more than just the PPL.
Good luck, whatever you choose.
FB