The P-line is the previous superlight with a new titanium rear triangle made for the new 4 speed derailleur.

The main frame is the same as on all previous Brompton and current A-line + C-line and the fork is the titanium fork of the previous superlight.

The only model which is fully different from previous Brompton is the T-line full titanium.

The problem of the Brompton isn't robustness, the bike can last decades with everyday use but stiffness, poor gearing and lack of comfort due to the relatively narrow and high pressure tires. There are some rumors saying the the T-line could accept wider tires but as far as I know no proof of it and anyway, there are no wide tires in ETRTO349, the only (slightly) wider tire are the Greenspeed Scorcher and Tru-blu 40x349.

That said, the Brompton is very pleasant to ride and very user friendly in everyday biking.